diff --git a/docs/make.jl b/docs/make.jl
index c6fe77a334..2ea90da038 100644
--- a/docs/make.jl
+++ b/docs/make.jl
@@ -29,6 +29,7 @@ pages = OrderedDict(
             "...improve performance with time series data" => "how_to/improve_ts_performance.md",
             "...serialize data to a JSON" => "how_to/serialize_data.md",
             "...reduce REPL printing" => "how_to/reduce_repl_printing.md",
+            "...migrate from version 3.0 to 4.0" => "how_to/migrating_to_psy4.md"
         ],
         "Explanation" =>
             Any[
diff --git a/docs/src/api/glossary.md b/docs/src/api/glossary.md
index dd094d7b26..de54a51e18 100644
--- a/docs/src/api/glossary.md
+++ b/docs/src/api/glossary.md
@@ -52,9 +52,12 @@
 
 ### H
 
-* *Horizon*: Is the count of discrete values in a forecast, all horizons in `PowerSystems.jl`
-  are represented with `Int`. For instance, many Day-ahead markets will have a forecast with a
-  horizon 24. See the article on [`Time Series Data`](@ref ts_data).
+* *Horizon*: Is the duration of all time steps in one forecast. As of PowerSystems.jl
+  version 4.0, all horizons in `PowerSystems.jl` are represented as a `Dates.Period`.
+  For instance, many Day-ahead markets will have an hourly-[resolution](@ref R) forecast
+  for the next day, which would have a horizon of `Dates.Hour(24)` or `Dates.Day(1)`. If the
+  forecast included the next day plus a 24-hour lookahead window, the horizon would be
+  `Dates.Hour(48)` or `Dates.Day(2)`. See the article on [`Time Series Data`](@ref ts_data).
 
 * *HVDC*: High-voltage DC
 
diff --git a/docs/src/how_to/migrating_to_psy4.md b/docs/src/how_to/migrating_to_psy4.md
new file mode 100644
index 0000000000..727a81dd12
--- /dev/null
+++ b/docs/src/how_to/migrating_to_psy4.md
@@ -0,0 +1,115 @@
+# [Migrating from version 3.0 to 4.0](@id psy4_migration)
+
+This guide outlines the code updates required to upgrade from PowerSystems.jl version 3.0
+to 4.0, which was released in June 2024 and includes breaking changes. These are:
+
+- [Renamed Types and Parameters](@ref)
+- [New and Eliminated Types](@ref)
+- [Updates to Energy Storage Parameters](@ref esr_migration)
+- [Hydropower `status` added](@ref)
+- [New Cost Functions](@ref)
+- [New Time Series Horizon Format](@ref)
+- [Minor Type Hierarchy Change](@ref)
+- [(Temporary) Use Version 3.0 for `HybridSystem` (+ new parameter)](@ref)
+
+## Renamed Types and Parameters
+Some `Types` and fields were renamed, which should require a trivial search and replace:
+
+Renamed `Types`:
+- `RenewableFix` is now named [`RenewableNonDispatch`](@ref)
+- `StaticReserve` is now named [`ConstantReserve`](@ref)
+- `StaticReserveGroup` is now named [`ConstantReserveGroup`](@ref)
+- `StaticReserveNonSpinning` is now named [`ConstantReserveNonSpinning`](@ref)
+- `PriorityCurrentLimiter` is now named [`PriorityOutputCurrentLimiter`](@ref)
+- `MagnitudeCurrentLimiter` is now named [`MagnitudeOutputCurrentLimiter`](@ref)
+- `InstantaneousCurrentLimiter` is now named [`InstantaneousOutputCurrentLimiter`](@ref)
+
+Renamed parameters:
+- The `rate` parameter is now named `rating` for subtypes of `Branch`, for
+    consistency with other Types. Affected Types are:
+    - [`Line`](@ref)
+    - [`MonitoredLine`](@ref)
+    - [`PhaseShiftingTransformer`](@ref)
+    - [`TapTransformer`](@ref)
+    - [`Transformer2W`](@ref)
+
+## New and Eliminated Types
+In addition to cost-related types detailed in [New Cost Functions](@ref), these new types
+have been added:
+- [`AreaInterchange`](@ref)
+- [`HybridOutputCurrentLimiter`](@ref)
+- [`SaturationOutputCurrentLimiter`](@ref)
+
+These types are no longer part of PowerSystems.jl, although there are future plans to rework
+some of them:
+- `RegulationDevice`
+- `Transfer`
+- `BatteryEMS`
+- `GenericBattery` (see [Updates to Energy Storage Parameters](@ref esr_migration))
+
+## [Updates to Energy Storage Parameters](@id esr_migration)
+[`EnergyReservoirStorage`](@ref) is now the default battery and energy storage model,
+replacing `GenericBattery`.
+
+There are also changes to the data fields compared to `GenericBattery` to improve clarity
+and modeling flexibility.
+
+New data fields:
+- `storage_capacity` for the maximum storage capacity (can be in units of,
+    e.g., MWh for batteries or liters for hydrogen)
+    - Example: 10000.0 for 10,000 liters hydrogen
+- `storage_level_limits` for the minimum and maximum allowable storage levels
+    on [0, 1], which can be used to model derates or other restrictions, such as
+    state-of-charge restrictions on battery cycling
+    - Example: Minimum of 0.2 and maximum of 1.0 to restrict the storage from dropping below
+        20% capacity to keep some reserve margin available at all times
+- `initial_storage_capacity_level` for the initial storage capacity level as
+    a ratio [0, 1.0] of `storage_capacity`
+    - Example: 0.5 to start the storage at 50% full
+- `conversion_factor` is the (optional) conversion factor of `storage_capacity` to MWh, if
+    different than 1.0 (i.e., no conversion is needed if the `storage_capacity` is in MWh)
+    - Example: 0.0005 for 0.5 kWh/l hydrogen
+
+Removed data fields:
+- `state_of_charge_limits` with units of p.u.-hr
+- `initial_energy` with units of p.u.-hr
+
+## Hydropower `status` added
+A new required parameter, `status`, was added to [`HydroEnergyReservoir`](@ref) and
+[`HydroPumpedStorage`](@ref), for the initial condition of the generator.
+- For [`HydroEnergyReservoir`](@ref), `status` can be `true` = on or `false` = off.
+- For [`HydroPumpedStorage`](@ref), `status` can be `PumpHydroStatus.PUMP`,
+    `PumpHydroStatus.GEN`, or `PumpHydroStatus.OFF`
+
+## New Cost Functions
+
+## New Time Series Horizon Format
+The [horizon](@ref H) for a forecast has changed from a **count** of time steps (as an
+    `Int`) to a **duration**, as a 
+    [`Dates.Period`](https://docs.julialang.org/en/v1/stdlib/Dates/#Period-Types)
+
+**Example day-ahead forecast:** A forecast with hourly [resolution](@ref R) for the next
+24 hours, with a new forecast available every 24 hours (i.e., 24-hour [interval](@ref I))
+- The horizon is now `Dates.Hour(24)` or `Dates.Day(1)`
+- Previously in version 3.0, the horizon would have been `24` for the 24 1-hour time-steps
+    in each forecast
+
+**Example hour-ahead forecast:** A forecast with 5-minute [resolution](@ref R) for the next
+1 hour, with a new forecast available every hour (i.e., 1-hour [interval](@ref I))
+- The horizon is now `Dates.Hour(1)`
+- Previously in version 3.0, the horizon would have been `12` for the 12 5-minute time-steps
+    in each forecast
+
+## Minor Type Hierarchy Change
+
+- [`ControllableLoad`](@ref) is now a subtype of [`StaticLoad`](@ref) rather than
+    `ElectricLoad`
+
+The vast majority of users are not expected to be impacted by this change.
+
+## (Temporary) Use Version 3.0 for `HybridSystem` (+ new parameter)
+The [`HybridSystem`](@ref) is currently not supported in the rest of the Sienna ecosystem,
+such as PowerSimulations.jl. To use `HybridSystem` in simulation, revert to version 3.0.
+There are plans to update `HybridSystem` for version 4.0, but they have not been completed.
+
+In addition, `HybridSystem` has a new required parameter: `interconnection_efficiency`
\ No newline at end of file
diff --git a/docs/src/index.md b/docs/src/index.md
index 130128e1f5..3c5e0d200d 100644
--- a/docs/src/index.md
+++ b/docs/src/index.md
@@ -4,6 +4,11 @@
 CurrentModule = PowerSystems
 ```
 
+!!! tip "Announcement"
+    PowerSystems.jl upgraded to version 4.0 in June 2024, which included breaking changes.
+    Visit the [v4.0 migration guide](@ref psy4_migration) for information on
+    how to update your existing code.
+
 !!! warning "Under Construction"
     The PowerSystems.jl is being actively being rewritten for version 4.0 using the new
     format described in [How To Use This Documentation](@ref). Your patience is appreciated