Previewed at WWDC, launching in the fall.
Apple Seeds First OS X 10.10.4 Yosemite Beta to Developers
The new beta, build 14E7f, can be downloaded from the Mac Developer Center.
It is not yet clear what's new in the 10.10.4 beta, but according to the release notes, the update "improves the stability, compatibility, and security of your Mac." Earlier OS X Yosemite updates, including OS X 10.10.1 and OS X 10.10.2 brought only minor changes to the operating system in the form of bug fixes and performance enhancements. OS X 10.10.4 may serve the same purpose.
OS X 10.10.3, released last week, was the first major update to OS X Yosemite that brought new features, including the Photos for OS X app, a redesigned emoji picker, and new diversified emoji.