diff --git a/.release-please-manifest.json b/.release-please-manifest.json
index b5ba789da..588e2d7b6 100644
--- a/.release-please-manifest.json
+++ b/.release-please-manifest.json
@@ -1,3 +1,3 @@
 {
-    "hugr-py": "0.3.1"
+    "hugr-py": "0.4.0"
 }
diff --git a/hugr-py/CHANGELOG.md b/hugr-py/CHANGELOG.md
index 605321721..95535c454 100644
--- a/hugr-py/CHANGELOG.md
+++ b/hugr-py/CHANGELOG.md
@@ -1,5 +1,17 @@
 # Changelog
 
+## [0.4.0](https://github.com/CQCL/hugr/compare/hugr-py-v0.3.1...hugr-py-v0.4.0) (2024-07-10)
+
+
+### ⚠ BREAKING CHANGES
+
+* **hugr-py:** dataclasses that derive from `ops.Custom` now have to be frozen, and instances cannot be mutated.
+
+### Features
+
+* **hugr-py:** `ops.Custom` is now a frozen dataclass ([94702d2](https://github.com/CQCL/hugr/commit/94702d2a9a9a6f4d311db6945ed2bee86b7bc46d))
+* **hugr-py:** move std extension types/ops in to `std` module ([#1288](https://github.com/CQCL/hugr/issues/1288)) ([7d82245](https://github.com/CQCL/hugr/commit/7d8224530ec4e70a7749505b379d7a4fe65f1168))
+
 ## [0.3.1](https://github.com/CQCL/hugr/compare/hugr-py-v0.3.0...hugr-py-v0.3.1) (2024-07-08)
 
 
diff --git a/hugr-py/pyproject.toml b/hugr-py/pyproject.toml
index 70528cd66..d5d82f0f1 100644
--- a/hugr-py/pyproject.toml
+++ b/hugr-py/pyproject.toml
@@ -13,7 +13,7 @@ classifiers = [
     "Topic :: Scientific/Engineering",
 ]
 name = "hugr"
-version = "0.3.1"
+version = "0.4.0"
 description = "Quantinuum's common representation for quantum programs"
 #keywords = []
 authors = ["TKET development team <tket-support@cambridgequantum.com>"]
diff --git a/hugr-py/src/hugr/__init__.py b/hugr-py/src/hugr/__init__.py
index 6a2de6ca8..048ac1423 100644
--- a/hugr-py/src/hugr/__init__.py
+++ b/hugr-py/src/hugr/__init__.py
@@ -4,4 +4,4 @@
 
 # This is updated by our release-please workflow, triggered by this
 # annotation: x-release-please-version
-__version__ = "0.3.1"
+__version__ = "0.4.0"