Preview at WWDC likely in June, followed by September launch.
Apple Seeds Second OS X 10.10.4 Yosemite Beta to Developers, First Public Beta Version
Developers can download the new beta, build 14E11f, through the Software Update mechanism in the Mac App Store or through the Mac Developer Center.
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.
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.