diff --git a/.github/workflows/build_upload_pypi_wheels.yml b/.github/workflows/build_upload_pypi_wheels.yml
index aa8d6a660..7243bd3f0 100644
--- a/.github/workflows/build_upload_pypi_wheels.yml
+++ b/.github/workflows/build_upload_pypi_wheels.yml
@@ -89,8 +89,8 @@ jobs:
 
           CIBW_REPAIR_WHEEL_COMMAND_MACOS: ""
 
-          CIBW_TEST_EXTRAS: "test,brille,phonopy_reader,matplotlib"
-          CIBW_TEST_COMMAND: "python {package}/tests_and_analysis/test/run_tests.py"
+          CIBW_TEST_EXTRAS: "test,brille,phonopy-reader,matplotlib"
+          CIBW_TEST_COMMAND: python {package}/tests_and_analysis/test/run_tests.py
 
         with:
           output-dir: wheelhouse
@@ -169,7 +169,7 @@ jobs:
 
       - name: Install from sdist
         shell: bash -l {0}
-        run: python -m pip install $(find dist -name 'euphonic-*.tar.gz')[matplotlib,phonopy_reader,brille,test]
+        run: python -m pip install $(find dist -name 'euphonic-*.tar.gz')[matplotlib,phonopy-reader,brille,test]
 
       - name: Checkout repository (for tests and test data)
         uses: actions/checkout@v4
diff --git a/.github/workflows/run_tests.yml b/.github/workflows/run_tests.yml
index d668cbd1d..374c64a6f 100644
--- a/.github/workflows/run_tests.yml
+++ b/.github/workflows/run_tests.yml
@@ -73,16 +73,13 @@ jobs:
         if: github.event_name != 'workflow_dispatch'
         env:
           TOX_SKIP_ENV: '.*?(py311).*?'
-          TOX_PARALLEL_NO_SPINNER: 1
         shell: bash -l {0}
-        run: python -m tox --parallel
+        run: python -m tox run-parallel
 
       - name: Run tests, workflow dispatch so test all Python versions
         if: github.event_name == 'workflow_dispatch'
-        env:
-          TOX_PARALLEL_NO_SPINNER: 1
         shell: bash -l {0}
-        run: python -m tox --parallel
+        run: python -m tox run-parallel
 
       - name: Upload test results
         if: always()
@@ -115,7 +112,7 @@ jobs:
           python -m pip install --upgrade pip
           python -m pip install -r tests_and_analysis/ci_requirements.txt
           python -m pip install -r doc/requirements.txt
-          python -m pip install .[matplotlib,phonopy_reader,brille]
+          python -m pip install .[matplotlib,phonopy-reader,brille]
       - name: Run Sphinx doctests
         working-directory: ./doc
         shell: bash -l {0}
diff --git a/.github/workflows/test_release.yml b/.github/workflows/test_release.yml
index b66179bf9..ee61504eb 100644
--- a/.github/workflows/test_release.yml
+++ b/.github/workflows/test_release.yml
@@ -15,12 +15,15 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
-      - uses: conda-incubator/setup-miniconda@v3
         with:
-          python-version: '3.10'
-          channels: conda-forge,defaults
-          channel-priority: true
-      - name: Install llvm on Macos
+          fetch-depth: 0 # Ensure tags are fetched for versioning
+      - uses: actions/setup-python@v5
+        with:
+          python-version: |
+              3.10
+              3.11
+              3.12
+      - name: Install llvm on MacOS
         if: startsWith(matrix.os, 'macos')
         shell: bash -l {0}
         env:
diff --git a/.readthedocs.yml b/.readthedocs.yml
index 8ec0978d5..b527e5116 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -23,5 +23,5 @@ python:
         path: .
         extra_requirements:
            - matplotlib
-           - phonopy_reader
+           - phonopy-reader
            - brille
diff --git a/build_utils/release_tox.ini b/build_utils/release_tox.ini
index 5c8a300a7..0ed260e2b 100644
--- a/build_utils/release_tox.ini
+++ b/build_utils/release_tox.ini
@@ -1,74 +1,52 @@
 [tox]
 # Use conda to set up the python environments to run in
-requires = tox-conda
+requires = tox>=4
 # The python environments to run the tests in
 envlist = pypi-py310-min,conda-py310-old-np,{pypi,conda}-{py310,py311,py312},pypisource-{py310,py312}
-# Skip the execution of setup.py as we do it with the correct version in commands_pre below
-skipsdist = True
+skip_install = True
 
 [testenv]
 changedir = tests_and_analysis/test
 test_command = python {toxinidir}/../tests_and_analysis/test/run_tests.py --report
 requirements_dir = {toxinidir}/../tests_and_analysis
+passenv =
+    EUPHONIC_VERSION
+# The name "extras" is reserved and has its own format with newlines:
+# we need a token to inject into custom commands_pre, so call it extras_str
+extras_str = [test,matplotlib,phonopy-reader,brille]
 
 # Test PyPI source distribution
 [testenv:pypisource-{py310,py312}]
-passenv = CC CC_LD LDFLAGS CPPFLAGS
-install_command = python -m pip install {opts} {packages}
-deps =
-    numpy
-    spglib  # Not interested here in whether source builds of spglib work
-    -r{[testenv]requirements_dir}/tox_requirements.txt
+passenv =
+    CC
+    CC_LD
+    LDFLAGS
+    CPPFLAGS
 commands_pre =
     python -m pip install \
     --force-reinstall \
-    'euphonic[matplotlib,phonopy_reader,brille]=={env:EUPHONIC_VERSION}' \
+    euphonic{[testenv]extras_str}=={env:EUPHONIC_VERSION} \
     --no-binary 'euphonic'
 commands = {[testenv]test_command}
 
-
 [testenv:pypi-{py310,py311,py312}]
-install_command = python -m pip install {opts} {packages}
-deps =
-    numpy
-    -r{[testenv]requirements_dir}/tox_requirements.txt
 commands_pre =
     python -m pip install \
     --force-reinstall \
-    'euphonic[matplotlib,phonopy_reader,brille]=={env:EUPHONIC_VERSION}' \
+    euphonic{[testenv]extras_str}=={env:EUPHONIC_VERSION} \
     --only-binary 'euphonic'
 commands = {[testenv]test_command}
 
-[testenv:pypi-py310-min-win-linux]
-install_command = python -m pip install --force-reinstall {opts} {packages}
-platform =
-    linux: linux
-    windows: win32
+[testenv:pypi-py310-min]
+platform = (linux)|(win32)
 deps =
-    numpy==1.24.0
+    numpy==1.24
+    -r{toxinidir}/../tests_and_analysis/tox_requirements.txt
 commands_pre =
     python -m pip install --force-reinstall \
-        -r{[testenv]requirements_dir}/minimum_euphonic_requirements.txt
-    python -m pip install --force-reinstall \
-        -r{[testenv]requirements_dir}/tox_requirements.txt
+        -r{toxinidir}/../tests_and_analysis/minimum_euphonic_requirements.txt
     python -m pip install \
-    'euphonic[matplotlib,phonopy_reader,brille]=={env:EUPHONIC_VERSION}' \
-    --only-binary 'euphonic'
-commands = {[testenv]test_command}
-
-[testenv:pypi-py310-min-mac]
-install_command = python -m pip install --force-reinstall {opts} {packages}
-platform =
-    mac: darwin
-deps =
-    numpy==1.24.0
-commands_pre =
-    python -m pip install --force-reinstall \
-        -r{[testenv]requirements_dir}/minimum_euphonic_requirements_mac.txt
-    python -m pip install --force-reinstall \
-        -r{[testenv]requirements_dir}/tox_requirements.txt
-    python -m pip install \
-    'euphonic[matplotlib,phonopy_reader,brille]=={env:EUPHONIC_VERSION}' \
+    euphonic{[testenv]extras_str}=={env:EUPHONIC_VERSION} \
     --only-binary 'euphonic'
 commands = {[testenv]test_command}
 
@@ -78,10 +56,13 @@ install_command = conda install {packages}
 conda_channels =
     conda-forge
     default
-conda_deps =
-    --file={[testenv]requirements_dir}/tox_requirements.txt
+extras =
+    test
+    matplotlib
+    phonopy-reader
+
 commands_pre =
-    conda install -c conda-forge euphonic={env:EUPHONIC_VERSION} matplotlib-base pyyaml h5py
+    conda install -c conda-forge euphonic={env:EUPHONIC_VERSION}
 # Brille not available on conda
 commands = {[testenv]test_command} -m "not brille"
 
@@ -93,10 +74,12 @@ install_command = conda install {packages}
 conda_channels =
     conda-forge
     default
-conda_deps =
-    --file={[testenv]requirements_dir}/tox_requirements.txt
+deps = numpy==1.24
+extras =
+    test
+    matplotlib
+    phonopy-reader
 commands_pre =
-    conda install numpy=1.22
-    conda install -c conda-forge euphonic={env:EUPHONIC_VERSION} matplotlib-base pyyaml h5py
+    conda install -c conda-forge euphonic={env:EUPHONIC_VERSION}
 # Brille not available on conda
 commands = {[testenv]test_command} -m "not brille"
diff --git a/doc/source/installation.rst b/doc/source/installation.rst
index 0682f9ff8..4bc67eea1 100644
--- a/doc/source/installation.rst
+++ b/doc/source/installation.rst
@@ -14,14 +14,14 @@ Euphonic has optional 'extensions' which require extra dependencies for certain
 They are listed below
 
 * **matplotlib** - required for plotting
-* **phonopy_reader** - required for reading Phonopy files
+* **phonopy-reader** - required for reading Phonopy files
 * **brille** - required for performing linear interpolation with the Brille library
 
 To install euphonic with all the dependencies run:
 
 .. code-block:: bash
 
-  pip install euphonic[matplotlib,phonopy_reader,brille]
+  pip install euphonic[matplotlib,phonopy-reader,brille]
 
 The dependencies can also be installed individually:
 
@@ -69,7 +69,7 @@ To install with all dependencies use:
 
 .. code-block:: bash
 
-  pip install .[matplotlib,phonopy_reader,brille]
+  pip install .[matplotlib,phonopy-reader,brille]
 
 If you don't require the extra dependencies, just use:
 
diff --git a/euphonic/readers/phonopy.py b/euphonic/readers/phonopy.py
index b1b0bc980..09c68ce50 100644
--- a/euphonic/readers/phonopy.py
+++ b/euphonic/readers/phonopy.py
@@ -18,7 +18,7 @@ def __init__(self):
             '\n\nCannot import yaml, h5py to read Phonopy files, maybe '
             'they are not installed. To install the optional '
             'dependencies for Euphonic\'s Phonopy reader, try:\n\n'
-            'pip install euphonic[phonopy_reader]\n')
+            'pip install euphonic[phonopy-reader]\n')
 
     def __str__(self):
         return self.message
diff --git a/pyproject.toml b/pyproject.toml
index 6aec791d4..dc1ade576 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -53,9 +53,11 @@ documentation = "https://euphonic.readthedocs.io/en/stable/"
 
 [project.optional-dependencies]
 matplotlib = ["matplotlib>=3.8.0"]
-phonopy_reader = ["h5py>=3.6.0", "PyYAML>=6.0"]
+phonopy_reader = ["h5py>=3.6.0", "PyYAML>=6.0"]  # Deprecated, will be removed in future versions.
+phonopy-reader = ["h5py>=3.6.0", "PyYAML>=6.0"]
 brille = ["brille>=0.7.0"]
-test = ["mock", "pytest~=7.0", "coverage", "pytest-mock", "pytest-lazy-fixture", "pytest-xvfb"]
+test = ["mock", "pytest~=7.0", "coverage", "pytest-mock", "pytest-lazy-fixture", "pytest-xvfb", "python-slugify"]
+ci = ["tox==4.32.2"]
 
 [project.scripts]
 euphonic-brille-convergence = "euphonic.cli.brille_convergence:main"
diff --git a/tests_and_analysis/ci_requirements.txt b/tests_and_analysis/ci_requirements.txt
index cea4f609d..974751142 100644
--- a/tests_and_analysis/ci_requirements.txt
+++ b/tests_and_analysis/ci_requirements.txt
@@ -1 +1 @@
-tox==3.28.0
+tox==4.23.2
diff --git a/tests_and_analysis/test/euphonic_test/test_force_constants_calculate_qpoint_phonon_modes.py b/tests_and_analysis/test/euphonic_test/test_force_constants_calculate_qpoint_phonon_modes.py
index 0462761c5..56abb442c 100644
--- a/tests_and_analysis/test/euphonic_test/test_force_constants_calculate_qpoint_phonon_modes.py
+++ b/tests_and_analysis/test/euphonic_test/test_force_constants_calculate_qpoint_phonon_modes.py
@@ -301,7 +301,7 @@ def test_with_use_c_false_doesnt_raise_error_or_warn(
             fc.calculate_qpoint_phonon_modes(get_test_qpts(), use_c=False)
         assert len(warn_record) == 0
 
-
+@pytest.mark.c_extension
 class TestForceConstantsCalculateQPointPhononModesWithCExtensionInstalled:
 
     @pytest.fixture
diff --git a/tests_and_analysis/test/pytest.ini b/tests_and_analysis/test/pytest.ini
index 0d0c1f49e..3c72d8828 100644
--- a/tests_and_analysis/test/pytest.ini
+++ b/tests_and_analysis/test/pytest.ini
@@ -4,3 +4,4 @@ markers =
     matplotlib: requires euphonic[matplotlib] extra to be installed
     brille: requires euphonic[brille] extra to be installed
     multiple_extras: requires any combination of the above extras e.g. phonopy_reader and matplotlib
+    c_extension: Requires c extension compiled
diff --git a/tox.ini b/tox.ini
index 1892f9cf5..9a168e3a5 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,99 +1,87 @@
 [tox]
+requires = tox>=4
 # The python environments to run the tests in
-envlist = py310,py311,py312,py310-{base,matplotlib,phonopy_reader,brille,all},py310-minrequirements-{mac,linux}
-# Skip the execution of setup.py as we do it with the correct arg in commands_pre below
-skipsdist = True
-whitelist_externals = git
+envlist = py310,py311,py312,py310-{base,matplotlib,phonopy-reader,brille,all,minrequirements-linux,minrequirements-mac,no-c}
+allowlist_externals = git
 
 [testenv]
+package = wheel
 changedir = tests_and_analysis/test
 test_command = python run_tests.py --report
-passenv = CC CC_LD LDFLAGS CPPFLAGS
-
-[testenv:{py310,py311,py312}]
+passenv = CC_LD
 install_command =
-    python -m pip install \
+    python -m pip install -v \
         --force-reinstall \
         --upgrade \
         --upgrade-strategy eager \
         {opts} \
         {packages}
-deps =
-    numpy
-    -r{toxinidir}/tests_and_analysis/tox_requirements.txt
-commands_pre =
-    python -m pip install \
-    --upgrade \
-    --upgrade-strategy eager \
-    '{toxinidir}[matplotlib,phonopy_reader,brille]'
-commands =
-    {[testenv]test_command} --cov
+extras =
+    test
+
+[testenv:{py310,py311,py312}]
+extras =
+    {[testenv]extras}
+    matplotlib
+    phonopy-reader
+    brille
+commands = {[testenv]test_command} --cov
 
 # Test with no extras
 [testenv:py310-base]
-install_command = {[testenv:py310]install_command}
-deps = {[testenv:py310]deps}
-commands_pre =
-    python -m pip install \
-        --upgrade \
-        --upgrade-strategy eager \
-        '{toxinidir}'
+extras = {[testenv]extras}
 commands = {[testenv]test_command} --cov -m "not (phonopy_reader or matplotlib or brille)"
 
 # Test with matplotlib extra only
 [testenv:py310-matplotlib]
-install_command = {[testenv:py310]install_command}
-deps = {[testenv:py310]deps}
-commands_pre =
-    python -m pip install \
-        --upgrade \
-        --upgrade-strategy eager \
-        '{toxinidir}[matplotlib]'
+extras =
+    {[testenv]extras}
+    matplotlib
 commands = {[testenv]test_command} --cov -m "matplotlib and not multiple_extras"
 
-# Test with phonopy_reader extra only
-[testenv:py310-phonopy_reader]
-install_command = {[testenv:py310]install_command}
-deps = {[testenv:py310]deps}
-commands_pre =
-    python -m pip install \
-        --upgrade \
-        --upgrade-strategy eager \
-        '{toxinidir}[phonopy_reader]'
+# Test with phonopy-reader extra only
+[testenv:py310-phonopy-reader]
+extras =
+    {[testenv]extras}
+    phonopy-reader
 commands = {[testenv]test_command} --cov -m "phonopy_reader and not multiple_extras"
 
 # Test with brille extra only
 [testenv:py310-brille]
-install_command = {[testenv:py310]install_command}
-deps = {[testenv:py310]deps}
-commands_pre =
-    python -m pip install \
-        --upgrade \
-        --upgrade-strategy eager \
-        '{toxinidir}[brille]'
+extras =
+    {[testenv]extras}
+    brille
 commands = {[testenv]test_command} --cov -m "brille and not multiple_extras"
 
 # Run remaining tests that require multiple extras
 [testenv:py310-all]
-install_command = {[testenv:py310]install_command}
-deps = {[testenv:py310]deps}
-commands_pre =
-    python -m pip install \
-        --upgrade \
-        --upgrade-strategy eager \
-        '{toxinidir}[phonopy_reader,matplotlib,brille]'
+extras =
+    {[testenv]extras}
+    matplotlib
+    phonopy-reader
+    brille
 commands =
     {[testenv]test_command} --cov -m multiple_extras
 
+[testenv:py310-no-c]
+install_command = {[testenv]install_command} -Csetup-args="-Dpython_only=true"
+platform = linux
+extras =
+    {[testenv]extras}
+    matplotlib
+    phonopy-reader
+    brille
+commands = {[testenv]test_command} -m 'not c_extension'
+
 [testenv:py310-minrequirements-linux]
-whitelist_externals = rm
+skip_install = True
+allowlist_externals = rm
 install_command =
     python -m pip install --force-reinstall {opts} {packages}
-platform =
-    linux: linux
+platform = linux
 deps =
     numpy==1.24.0
-    {[testenv:py310]deps}
+    -r{toxinidir}/tests_and_analysis/tox_requirements.txt
 commands_pre =
     python -m pip install --force-reinstall \
         -r{toxinidir}/tests_and_analysis/minimum_euphonic_requirements.txt
@@ -101,18 +89,18 @@ commands_pre =
         -r{toxinidir}/tests_and_analysis/tox_requirements.txt
     # Force rebuild of euphonic extension to avoid Numpy clash
     rm -rf {toxinidir}/build
-    python -m pip install '{toxinidir}[matplotlib,phonopy_reader,brille]'
+    python -m pip install '{toxinidir}[matplotlib,phonopy-reader,brille]'
 commands = {[testenv]test_command}
 
 [testenv:py310-minrequirements-mac]
-whitelist_externals = rm
+skip_install = True
+allowlist_externals = rm
 install_command =
     python -m pip install --force-reinstall {opts} {packages}
-platform =
-    macos: darwin
+platform = darwin
 deps =
     numpy==1.24.0
-    {[testenv:py310]deps}
+    -r{toxinidir}/tests_and_analysis/tox_requirements.txt
 commands_pre =
     python -m pip install --force-reinstall \
         -r{toxinidir}/tests_and_analysis/minimum_euphonic_requirements_mac.txt
@@ -120,5 +108,5 @@ commands_pre =
         -r{toxinidir}/tests_and_analysis/tox_requirements.txt
     # Force rebuild of euphonic extension to avoid Numpy clash
     rm -rf {toxinidir}/build
-    python -m pip install '{toxinidir}[matplotlib,phonopy_reader,brille]'
+    python -m pip install '{toxinidir}[matplotlib,phonopy-reader,brille]'
 commands = {[testenv]test_command}