Update CHANGELOG in preparation for release.

Author: tleonhardt

CHANGELOG.md

@@ -1,4 +1,4 @@
-## 4.0.0 (June TBD, 2026)
+## 4.0.0 (June 5, 2026)
 
 ### Summary
 

README.md

@@ -26,7 +26,7 @@ of cmd to make your life easier and eliminates much of the boilerplate code whic
 when using cmd.
 
 > :warning: **If you are upgrading from an older version of `cmd2`, both `3.x` and `4.x` have some
-> significant backwards incompatibilities from version `2.x`. Please see the
+> significant backwards incompatibilities from previous versions. Please see the
 > [CHANGELOG](./CHANGELOG.md) for info on what has changed and the
 > [Migration Guide](https://cmd2.readthedocs.io/en/latest/upgrades/) for tips on upgrading from an
 > older version of `cmd2` to `4.x`**