Bumping CHANGELOG.md for v4 release

Author: caseyamcl

CHANGELOG.md

@@ -4,7 +4,7 @@ All notable changes to this project are documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## UNRELEASED (v4)
+## [4.0.0] - 2024-12-20
 ### Added
 - `SluggerInterface` interface for new `UniqueSlugger` logic
 ### Changed