doc: update for v0.4.0 release

Author: lanej

CHANGELOG.md

@@ -1,8 +1,8 @@
 # Changelog
 
-## [Unreleased](https://github.com/EasyPost/toggles/tree/HEAD)
+## [v0.4.0](https://github.com/EasyPost/toggles/tree/v0.4.0)
 
-[Full Changelog](https://github.com/EasyPost/toggles/compare/v0.3.0...HEAD)
+[Full Changelog](https://github.com/EasyPost/toggles/compare/v0.3.0...v0.4.0)
 
 **Merged pull requests:**