Preview at WWDC likely in June, followed by September launch.
Apple Seeds Fourth OS X 10.10.4 Yosemite Beta to Developers, Public Beta Testers
The new beta, build 14E26a, can be downloaded through the Software Update mechanism in the Mac App Store or through the Mac Developer Center.
OS X 10.10.3 brought several consumer-facing changes including the new Photos for OS X app, a redesigned emoji picker, and diversified emoji, but OS X 10.10.4 appears to be an under-the-hood update that brings performance enhancements and bug fixes. Thus far, OS X 10.10.4 betas have not included outward facing design changes or feature additions.
The first two updates to OS X Yosemite, OS X 10.10.1 and OS X 10.10.2, were also minor behind-the-scenes updates that improved performance through bug fixes and enhancements.