diff --git a/.travis.yml b/.travis.yml
index 5737015..63f7b35 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,3 @@
 language: node_js
 node_js:
-  - "0.12"
-  - "0.10"
-  - "iojs"
+  - "4"
diff --git a/README.md b/README.md
index f074de3..69deea9 100644
--- a/README.md
+++ b/README.md
@@ -5,6 +5,9 @@
 
 *A node [gitlab-ci-runner](https://github.com/gitlabhq/gitlab-ci-runner)*
 
+`gcr` v4.x will only support node v4.x+. To use `gcr` with an older version
+of node, please use `gcr` v3.x
+
 ## Install
 
 ```bash
diff --git a/lib/build.js b/lib/build.js
index c3f090f..46f5c7a 100644
--- a/lib/build.js
+++ b/lib/build.js
@@ -1,15 +1,17 @@
-var gcr = require('./gcr')
-  , spawn = require('child_process').spawn
-  , rimraf = require('rimraf')
-  , slide = require('slide')
-  , chain = slide.chain
-  , fs = require('fs')
-  , log = require('npmlog')
-  , path = require('path')
-  , util = require('util')
-  , mkdirp = require('mkdirp')
-  , argsplit = require('argsplit')
-  , EE = require('events').EventEmitter
+'use strict'
+
+const gcr = require('./gcr')
+    , spawn = require('child_process').spawn
+    , rimraf = require('rimraf')
+    , slide = require('slide')
+    , chain = slide.chain
+    , fs = require('fs')
+    , log = require('npmlog')
+    , path = require('path')
+    , util = require('util')
+    , mkdirp = require('mkdirp')
+    , argsplit = require('argsplit')
+    , EE = require('events')
 
 module.exports = Build
 
@@ -32,37 +34,37 @@ util.inherits(Build, EE)
 
 Build.prototype.run = function() {
   this.state = 'running'
-  var self = this
-  this.shouldClone(function(should) {
+  const self = this
+  this.shouldClone((should) => {
     if (!should) {
-      self.fetch(function(err) {
-        if (err) return self.emit('done', false)
+      this.fetch((err) => {
+        if (err) return this.emit('done', false)
         runCommand(0)
       })
     } else {
-      rimraf(self.projectDir, function(err) {
+      rimraf(self.projectDir, (err) => {
         if (err) {
-          self.state = 'failed'
-          self.update(function() {
-            self.emit('done', false)
+          this.state = 'failed'
+          this.update(() => {
+            this.emit('done', false)
           })
           return
         }
-        self.clone(function(err) {
-          if (err) return self.emit('done', false)
+        this.clone((err) => {
+          if (err) return this.emit('done', false)
           runCommand(0)
         })
       })
     }
   })
 
-  var cmds = this.opts.commands
-  var len = cmds.length
-  var dir = this.projectDir
+  const cmds = this.opts.commands
+  const len = cmds.length
+  const dir = this.projectDir
   function runCommand(idx) {
     if (idx < len) {
       log.verbose('[build]', 'running command', idx+1, 'of', len)
-      var cmd = cmds[idx]
+      const cmd = cmds[idx]
       self.runCommand(cmd, dir, function(err) {
         if (err) return self.emit('done', false)
         runCommand(idx+1)
@@ -77,20 +79,19 @@ Build.prototype.run = function() {
 }
 
 Build.prototype.clone = function(cb) {
-  var self = this
-  var cmd = ['clone', this.opts.repo_url, 'project-'+this.opts.project_id]
-  var dir = this.projectDir
-  this.gitCommand(cmd, this.buildDir, function(err) {
+  const cmd = ['clone', this.opts.repo_url, 'project-'+this.opts.project_id]
+  const dir = this.projectDir
+  this.gitCommand(cmd, this.buildDir, (err) => {
     if (err) return cb && cb(err)
-    self.gitCommand(['checkout', self.opts.ref], dir, cb)
+    this.gitCommand(['checkout', this.opts.ref], dir, cb)
   })
 }
 
 Build.prototype.fetch = function(cb) {
-  var self = this
+  const self = this
 
-  var dir = this.projectDir
-  var repoUrl = this.opts.repo_url
+  const dir = this.projectDir
+  const repoUrl = this.opts.repo_url
 
   function resetHard(cb) {
     self.gitCommand(['reset', '--hard'], dir, cb)
@@ -125,11 +126,10 @@ Build.prototype.fetch = function(cb) {
 }
 
 Build.prototype.shouldClone = function(cb) {
-  var self = this
-  var d = path.join(this.projectDir, '.git')
-  fs.exists(d, function(e) {
+  const d = path.join(this.projectDir, '.git')
+  fs.exists(d, (e) => {
     if (!e) return cb(true)
-    return cb(!self.opts.allow_git_fetch)
+    return cb(!this.opts.allow_git_fetch)
   })
 }
 
@@ -138,9 +138,9 @@ Build.prototype.append = function(str) {
 }
 
 Build.prototype.runCommand = function(cmd, dir, cb) {
-  var self = this
+  const self = this
   if ('function' === typeof dir) cb = dir, dir = process.cwd()
-  var env = {
+  const env = {
     CI_SERVER: true
   , CI_SERVER_NAME: 'GitLab CI'
   , CI_SERVER_VERSION: null
@@ -159,7 +159,7 @@ Build.prototype.runCommand = function(cmd, dir, cb) {
 
   util._extend(env, process.env)
 
-  var opts = {
+  const opts = {
     env: env
   , cwd: dir
   , timeout: this.opts.timeout
@@ -171,57 +171,73 @@ Build.prototype.runCommand = function(cmd, dir, cb) {
   }
 
   log.verbose('[builder]', 'cmd', cmd)
-  this.append(util.format('\n%s\n', cmd))
+  this.append(`\n${cmd}\n`)
 
-  var child = spawn('/bin/bash', ['-c', fixedCmd.join(' ')], opts)
+  const child = spawn('/bin/bash', ['-c', fixedCmd.join(' ')], opts)
   var timedout = false
-  var timer = setTimeout(function() {
+  var timer = setTimeout(() => {
     timedout = true
     child.kill()
-    self.append('\n** TIMEOUT **\n')
+    this.append('\n** TIMEOUT **\n')
   }, this.opts.timeout)
-  child.stderr.on('data', function(d) {
-    var data = d.toString()
+
+  child.stderr.on('data', (d) => {
+    const data = d.toString()
     log.silly('[builder]', 'stderr', data)
-    self.append(data)
+    this.append(data)
   })
-  child.stdout.on('data', function(d) {
-    var data = d.toString()
+
+  child.stdout.on('data', (d) => {
+    const data = d.toString()
     log.silly('[builder]', 'stdout', data)
-    self.append(data)
+    this.append(data)
+  })
+
+  child.on('error', (err) => {
+    if (timer) {
+      clearTimeout(timer)
+      timer = null
+    }
+    const e = new Error(`Command: [${cmd}] failed ${err.message}`)
+    this.append(e)
+    log.error(e, err)
+    this.state = 'failed'
+    this.update(function() {
+      cb && cb(e)
+    })
   })
-  child.on('close', function(code) {
+
+  child.on('close', (code) => {
     if (timer) {
       clearTimeout(timer)
       timer = null
     }
     if (code !== 0) {
-      var msg = timedout
+      const msg = timedout
         ? 'process timedout'
-        : 'process exited with code: '+code
-      var e = new Error(msg)
-      self.append(util.format(
-        'Command: [%s] exited with code: %d timedout: %s', cmd, code,
-          timedout ? 'yes' : 'no'
-      ))
+        : `process exited with code: ${code}`
+
+      const e = new Error(msg)
+      const to = timedout ? 'yes' : 'no'
+      this.append(`Command: ${cmd} exited with code: ${code} timedout: ${to}`)
       e.command = cmd
       e.opts = opts
-      self.state = 'failed'
-      self.update(function() {
+      this.state = 'failed'
+      this.update(function() {
         cb && cb(e)
       })
       return
     }
-    self.update(function() {
+    this.update(function() {
       cb && cb()
     })
   })
 }
 
 Build.prototype.gitCommand = function(cmd, dir, cb) {
-  var self = this
+  const self = this
   if ('function' === typeof dir) cb = dir, dir = process.cwd()
-  var env = {
+  const env = {
     CI_SERVER: true
   , CI_SERVER_NAME: 'GitLab CI'
   , CI_SERVER_VERSION: null
@@ -240,32 +256,49 @@ Build.prototype.gitCommand = function(cmd, dir, cb) {
 
   util._extend(env, process.env)
 
-  var opts = {
+  const opts = {
     env: env
   , cwd: dir
   , timeout: this.opts.timeout
   }
 
   log.verbose('[builder]', 'cmd', cmd)
-  self.append(util.format('\n%s %s\n', self.git, cmd.join(' ')))
-  var child = spawn(this.git, cmd, opts)
+  self.append(`\n${self.git} ${cmd.join(' ')}\n`)
+  const child = spawn(this.git, cmd, opts)
   var timedout = false
-  var timer = setTimeout(function() {
+  var timer = setTimeout(() => {
     timedout = true
     child.kill()
-    self.append('\n** TIMEOUT **\n')
+    this.append('\n** TIMEOUT **\n')
   }, this.opts.timeout)
-  child.stderr.on('data', function(d) {
-    var data = d.toString()
+
+  child.stderr.on('data', (d) => {
+    const data = d.toString()
     log.silly('[builder]', 'stderr', data)
-    self.append(data)
+    this.append(data)
   })
-  child.stdout.on('data', function(d) {
-    var data = d.toString()
+
+  child.stdout.on('data', (d) => {
+    const data = d.toString()
     log.silly('[builder]', 'stdout', data)
-    self.append(data)
+    this.append(data)
   })
-  child.on('exit', function(code) {
+
+  child.on('error', (err) => {
+    if (timer) {
+      clearTimeout(timer)
+      timer = null
+    }
+    const e = new Error(`Command: [${cmd}] failed ${err.message}`)
+    this.append(e)
+    log.error(e, err)
+    this.state = 'failed'
+    this.update(function() {
+      cb && cb(e)
+    })
+  })
+
+  child.on('exit', (code) => {
     if (timer) {
       clearTimeout(timer)
       timer = null
@@ -273,27 +306,26 @@ Build.prototype.gitCommand = function(cmd, dir, cb) {
     if (code !== 0) {
       var msg = timedout
         ? 'process timedout'
-        : 'process exited with code: '+code
-      var e = new Error(msg)
-      self.append(util.format(
-        'Command: [%s %s] exited with code: %d timedout: %s', self.git,
-          cmd.join(' '), code, timedout ? 'yes' : 'no'
-      ))
+        : `process exited with code: ${code}`
+      const e = new Error(msg)
+      const to = timedout ? 'yes' : 'no'
+      this.append(`Command: [${self.git} ${cmd.join(' ')}] exited with code: ` +
+        `${code} timedout: ${to}`)
       e.command = cmd
       e.opts = opts
-      self.state = 'failed'
-      self.update(function() {
+      this.state = 'failed'
+      this.update(function() {
         cb && cb(e)
       })
       return
     }
-    self.update(function() {
+    this.update(function() {
       cb && cb()
     })
   })
 }
 
 Build.prototype.update = function(cb) {
-  var id = this.opts.id
-  gcr.client.updateBuild(id, this.state, this.output, cb)
+  const id = this.opts.id
+  gcr.client.updateBuild(+id, this.state, this.output, cb)
 }
diff --git a/lib/client.js b/lib/client.js
index 55b125b..133d7d9 100644
--- a/lib/client.js
+++ b/lib/client.js
@@ -1,10 +1,13 @@
-var request = require('request')
-  , gcr = require('./gcr')
-  , util = require('util')
-  , log = require('npmlog')
-  , url = require('url')
+'use strict'
 
-var proxyServer = process.env.HTTPS_PROXY
+const request = require('request')
+    , gcr = require('./gcr')
+    , util = require('util')
+    , log = require('npmlog')
+    , url = require('url')
+    , urlJoin = require('url-join')
+
+const proxyServer = process.env.HTTPS_PROXY
   || process.env.https_proxy
   || process.env.HTTP_PROXY
   || process.env.http_proxy
@@ -25,9 +28,8 @@ Client.prototype.apiUrl = function() {
 }
 
 Client.prototype.updateBuild = function(id, state, trace, cb) {
-  var self = this
   log.info('[client]', 'submitting build %d to coordinator...', id)
-  var opts = {
+  const opts = {
     body: {
       state: state
     , trace: trace
@@ -38,9 +40,9 @@ Client.prototype.updateBuild = function(id, state, trace, cb) {
   }
 
   log.verbose('[client]', 'update build', id, state)
-  opts.uri = url.resolve( self.apiUrl()
-                        , util.format('/api/v1/builds/%d.json', id)
-                        )
+  opts.uri = urlJoin( this.apiUrl()
+                    , util.format('/api/v1/builds/%d.json', id)
+                    )
   log.http('PUT', opts.uri)
   request.put(opts, function(err, res, body) {
     if (err) return cb && cb(err)
@@ -56,8 +58,7 @@ Client.prototype.updateBuild = function(id, state, trace, cb) {
 }
 
 Client.prototype.registerRunner = function(pubkey, token, cb) {
-  var self = this
-  var opts = {
+  const opts = {
     body: {
       public_key: pubkey
     , token: token
@@ -66,7 +67,7 @@ Client.prototype.registerRunner = function(pubkey, token, cb) {
   , strictSSL: gcr.config.get('strictSSL')
   }
 
-  opts.uri = url.resolve(self.apiUrl(), '/api/v1/runners/register.json')
+  opts.uri = urlJoin(this.apiUrl(), '/api/v1/runners/register.json')
   log.http('POST', opts.uri)
   request.post(opts, function(err, res, body) {
     if (err) return cb && cb(err)
@@ -82,10 +83,9 @@ Client.prototype.registerRunner = function(pubkey, token, cb) {
 }
 
 Client.prototype.getBuild = function(cb) {
-  var self = this
   log.info('[client]', 'checking for builds...')
 
-  var opts = {
+  const opts = {
     body: {
       token: gcr.config.get('token')
     }
@@ -93,12 +93,12 @@ Client.prototype.getBuild = function(cb) {
   , strictSSL: gcr.config.get('strictSSL')
   }
 
-  opts.uri = url.resolve(self.apiUrl(), '/api/v1/builds/register.json')
-  request.post(opts, function(err, res, body) {
+  opts.uri = urlJoin(this.apiUrl(), '/api/v1/builds/register.json')
+  request.post(opts, (err, res, body) => {
     if (err) return cb && cb(err)
     log.http(res.statusCode, opts.uri)
     if (res.statusCode === 201) {
-      var o = {
+      const o = {
         id: body.id
       , project_id: body.project_id
       , commands: body.commands.replace(/\r\n/g, '\n').split('\n')
diff --git a/lib/config.default.js b/lib/config.default.js
index 560522b..033f0dd 100644
--- a/lib/config.default.js
+++ b/lib/config.default.js
@@ -1,10 +1,12 @@
-var path = require('path')
-  , os = require('os')
-  , isWin = os.platform() === 'win32'
-  , homedir = require('os-homedir')
+'use strict'
+
+const path = require('path')
+    , os = require('os')
+    , isWin = os.platform() === 'win32'
+    , homedir = os.homedir
 
 module.exports = function(parsed) {
-  var o = {}
+  const o = {}
   if (parsed.url) o.url = parsed.url
   if (parsed.token) o.token = parsed.token
   if (parsed.registToken) o.registToken = parsed.registerToken
diff --git a/lib/gcr.js b/lib/gcr.js
index 4e132b8..1128333 100644
--- a/lib/gcr.js
+++ b/lib/gcr.js
@@ -1,15 +1,17 @@
-var EventEmitter = require('events').EventEmitter
-  , gcr = new EventEmitter
-  , log = require('npmlog')
-  , fs = require('fs')
-  , nconf = require('nconf')
-  , mkdirp = require('mkdirp')
-  , path = require('path')
-  , home = require('os-homedir')()
-  , confFile = path.join(home, '.config', 'gcr.json')
-  , slide = require('slide')
-  , chain = slide.chain
-  , which = require('which')
+'use strict'
+
+const EventEmitter = require('events')
+    , gcr = new EventEmitter
+    , log = require('npmlog')
+    , fs = require('fs')
+    , nconf = require('nconf')
+    , mkdirp = require('mkdirp')
+    , path = require('path')
+    , home = require('os-homedir')()
+    , confFile = path.join(home, '.config', 'gcr.json')
+    , slide = require('slide')
+    , chain = slide.chain
+    , which = require('which')
 
 log.heading = 'gcr'
 
diff --git a/lib/runner.js b/lib/runner.js
index bfb214d..cdea92d 100644
--- a/lib/runner.js
+++ b/lib/runner.js
@@ -1,8 +1,10 @@
-var gcr = require('./gcr')
-  , util = require('util')
-  , EventEmitter = require('events').EventEmitter
-  , log = require('npmlog')
-  , Build = require('./build')
+'use strict'
+
+const gcr = require('./gcr')
+    , util = require('util')
+    , EventEmitter = require('events')
+    , log = require('npmlog')
+    , Build = require('./build')
 
 module.exports = Runner
 
@@ -11,9 +13,9 @@ function Runner() {
     return new Runner()
   EventEmitter.call(this)
   // { buildId: projectId }
-  this.builds = {}
+  this.builds = new Map()
   // { buildId: buildData }
-  this.queue = {}
+  this.queue = new Map()
   this.interval = null
 }
 
@@ -27,14 +29,10 @@ Runner.prototype.start = function() {
   }, 5000)
 }
 
+// id must always be a number
 Runner.prototype.projectIsRunning = function(id) {
-  var builds = Object.keys(this.builds)
-    , len = builds.length
-
-  for (var i=0; i<len; i++) {
-    var key = builds[i]
-    var proj = this.builds[key]
-    if (+proj === +id) return true
+  for (let proj of this.builds.values()) {
+    if (proj === id) return true
   }
 
   return false
@@ -53,7 +51,7 @@ Runner.prototype.getBuild = function() {
     if (self.projectIsRunning(projectId)) {
       // We are already running a build for this project
       // queue it
-      self.queue[buildId] = data
+      self.queue.set(buildId, data)
     } else {
       // Go ahead and start the build
       self.runBuild(data)
@@ -64,23 +62,17 @@ Runner.prototype.getBuild = function() {
 Runner.prototype.checkQueue = function() {
   var self = this
   var queue = this.queue
-  var queuedProjects = Object.keys(queue).map(function(build) {
-    return queue[build]
-  })
-  queuedProjects.forEach(function(proj) {
-    if (self.projectIsRunning(proj.project_id)) {
-      // We are already running a build for this project
-      // It is already queued so do nothing
+  this.queue.forEach(function(key, val) {
+    if (self.projectIsRunning(+val.project_id))
       return
-    }
-    self.runBuild(proj)
-    delete self.queue[proj.id]
+    self.runBuild(val)
+    self.queue.delete(+val.id)
   })
 }
 
 Runner.prototype.runBuild = function(data) {
   var self = this
-  this.builds[data.id] = data.project_id
+  this.builds.set(+data.id, +data.project_id)
   var build = new Build(data)
   build.on('done', function(success) {
     if (success) {
@@ -89,8 +81,8 @@ Runner.prototype.runBuild = function(data) {
       log.error('[runner]', 'build failed [%s]', build.opts.id)
     }
     // cleanup
-    delete self.builds[build.opts.id]
-    delete build
+    self.builds.delete(+build.opts.id)
+    build = null
     self.checkQueue()
   })
   build.run()
diff --git a/lib/utils.js b/lib/utils.js
index 23ec2d8..629f460 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -1,11 +1,13 @@
-var log = require('npmlog')
-  , gcr = require('./gcr')
-  , exec = require('child_process').exec
-  , path = require('path')
-  , os = require('os')
-  , isWin = os.platform() === 'win32'
-  , fs = require('fs')
-  , utils = exports
+'use strict'
+
+const log = require('npmlog')
+    , gcr = require('./gcr')
+    , exec = require('child_process').exec
+    , path = require('path')
+    , os = require('os')
+    , isWin = os.platform() === 'win32'
+    , fs = require('fs')
+    , utils = exports
 
 utils.genSSHKey = function(fp, cb) {
   var k = isWin
diff --git a/package.json b/package.json
index 51d7b30..cec241c 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "gcr",
-  "version": "2.0.1",
+  "version": "3.0.0",
   "description": "A node gitlab-ci-runner",
   "main": "./lib/gcr.js",
   "bin": {
@@ -18,6 +18,7 @@
     "request": "~2.53.0",
     "rimraf": "~2.2.6",
     "slide": "~1.1.5",
+    "url-join": "0.0.1",
     "which": "~1.0.5"
   },
   "devDependencies": {
diff --git a/test/fixtures/server.js b/test/fixtures/server.js
index e69c437..4854b7d 100644
--- a/test/fixtures/server.js
+++ b/test/fixtures/server.js
@@ -7,6 +7,8 @@ function handle(req, res) {
   if (req.method === 'PUT') return handlePUT(req, res)
   if (url === '/api/v1/runners/register.json') {
     return handleRegister(req, res)
+  } else if (url === '/ci/api/v1/runners/register.json') {
+    return handleRegister(req, res)
   }
 
   var body = ''
diff --git a/test/gcr.js b/test/gcr.js
index d3eadad..57de61a 100644
--- a/test/gcr.js
+++ b/test/gcr.js
@@ -44,7 +44,7 @@ describe('gcr', function() {
   it('should allow loading', function(done) {
     gcr.loaded.should.be.false
     gcr.load({
-      url: 'http://127.0.0.1:' + port
+      url: 'http://127.0.0.1:' + port + '/ci'
     , token: 'biscuits'
     , buildDir: '/tmp/gcr-builds'
     , npm: true
@@ -150,7 +150,7 @@ describe('gcr', function() {
       should.exist(gcr.client)
     })
 
-    it('apuUrl() should return gcr.config.get(\'url\')', function() {
+    it('apiUrl() should return gcr.config.get(\'url\')', function() {
       gcr.client.apiUrl().should.equal(gcr.config.get('url'))
     })
 
@@ -225,12 +225,12 @@ describe('gcr', function() {
 
     describe('projectIsRunning', function() {
       it('should return true if a build exists', function() {
-        gcr.runner.builds = { 1: 1 }
+        gcr.runner.builds = new Map([[1, 1]])
         gcr.runner.projectIsRunning(1).should.be.true
       })
 
       it('should return false if a build does not exist', function() {
-        gcr.runner.builds = {}
+        gcr.runner.builds = new Map()
         gcr.runner.projectIsRunning(1).should.be.false
       })
     })