No breaking changes but lots of UI and UX improvements for editors
Version 4.0 of WordPress, named “Benny” in honor of jazz clarinetist and bandleader Benny Goodman, is available for download or update in your WordPress dashboard.
Don’t be afraid of breaking changes. It seems this 4.0 release should really be a 3.10 release because there are no breaking changes. See http://semver.org/ for proper semantic versioning.
Improvements cover three main areas:
1. Better editing experience including a fixed right bar, auto embedding of videos and tweets, and more intuitive editor.
2. An improved plugin browser that will speed up the process of finding good plugins.
3. A better experience for uploading, editing, and viewing media library assets.
Read more from the source: WordPress News