Previewed at WWDC, launching in the fall.
Apple Seeds Third OS X 10.10.4 Yosemite Beta to Developers and Public Beta Testers
The beta, build 14E17e, can be downloaded through the Software Update mechanism in the Mac App Store or through the Mac Developer Center. Apple is asking developers to focus on Photos, Migration, and Arabic and Hebrew languages.
OS X 10.10.3 introduced several consumer-facing changes including the new Photos for OS X app, a redesigned emoji picker, new diversified emoji, and more, 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.