New features include: Uploads repository enhancements. A new details preview makes viewing and editing any amount of media in sequence a snap. Enhancements when embedding videos — embedding has become a visual experience. The editor shows a true preview of your embedded content, saving you time and giving you confidence. Writing and editing is smoother and more immersive with an editor that expands to fit your content as you write, and keeps the formatting tools available at all times. WordPress 4.0 makes it easier to find the right plugins in the WordPress plugin directory, with new metrics, improved search, and a more visual browsing experience.
Read more and download version 4.0 here - http://wordpress.org/news/2014/09/benny/