An official announcement from Mozilla has yet to make its way to the official beta site, but we can report that Mozilla have selected the 3rd build candidate as beta 4.
There will be one final beta version in September, before Mozilla signs off on Firefox 4 which is scheduled to arrive in October.
This version includes
"Tab Groups" "Panorama" thanks -Aaron (Mozilla QA) which has been renamed from Tab Candy, it allows you to group specific tabs into groups, for example: work, fun and anything else you could think of. The feature can now be accessed with the keyboard shortcut Ctrl-Space.
Direct2D rendering is still not activated by default on Windows, because as revealed last week Mike Shaver, Mozillas vice president of engineering, sent a tweet stating "b4 wont have D2D on by default after all; just not quite ready to send it to that many users. soon!"
There are no release notes available yet, but you can pick up language and system specific versions at the usual spot on Mozillas official FTP site.