Update version to 26.4.0 and changelog (#5455)

Author: rtimms

CHANGELOG.md

@@ -1,5 +1,7 @@
 # [Unreleased](https://github.com/pybamm-team/PyBaMM/)
 
+# [v26.4.0](https://github.com/pybamm-team/PyBaMM/tree/v26.4.0) - 2026-04-22
+
 ## Features
 
 - Improved the codegen performance of averaging over spatial domains. ([#5448](https://github.com/pybamm-team/PyBaMM/pull/5448))
@@ -17,8 +19,6 @@
 - Fixed termination hashes causing duplicate experiment steps to be built. ([#5453](https://github.com/pybamm-team/PyBaMM/pull/5453))
 - Fixed a regression causing inaccurate initial guesses at experiment step transitions. ([#5452](https://github.com/pybamm-team/PyBaMM/pull/5452))
 
-## Breaking changes
-
 # [v26.3.1](https://github.com/pybamm-team/PyBaMM/tree/v26.3.1) - 2026-04-10
 
 ## Bug fixes

CITATION.cff

@@ -24,6 +24,6 @@ keywords:
   - "expression tree"
   - "python"
   - "symbolic differentiation"
-version: "26.3.1"
+version: "26.4.0"
 repository-code: "https://github.com/pybamm-team/PyBaMM"
 title: "Python Battery Mathematical Modelling (PyBaMM)"