Fixup 4.0.0 changelog

Signed-off-by: Jared Ledvina <jared@techsmix.net>

CHANGELOG.md

@@ -152,7 +152,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/).
 
 First tagged release, starting at 1.0.0 since the project can be considered stable at this point.
 
-[Unreleased]: https://github.com/sensu/sensu-ansible/compare/3.0.0...HEAD
+[Unreleased]: https://github.com/sensu/sensu-ansible/compare/4.0.0...HEAD
+[4.0.0]: https://github.com/sensu/sensu-ansible/compare/3.0.0...4.0.0
 [3.0.0]: https://github.com/sensu/sensu-ansible/compare/2.7.0...3.0.0
 [2.7.0]: https://github.com/sensu/sensu-ansible/compare/2.6.0...2.7.0
 [2.6.0]: https://github.com/sensu/sensu-ansible/compare/2.5.0...2.6.0