Previewed at WWDC, launching in the fall.
Apple Seeds Fifth OS X 10.10.4 Yosemite Beta to Developers and Public Beta Testers
The new beta, build 14E33b, can 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, but the last beta introduced a significant behind-the-scenes update that may improve networking performance for users who have experienced issues with OS X Yosemite.
Discoveryd, a somewhat problematic networking process that has been linked to multiple bugs in OS X 10.10, was removed in favor of the older bug-free mDNSresponder.
It is not clear when Apple plans to release OS X 10.10.4 to the public, but the fact that we've now received five betas suggests we may be nearing the end of the beta testing period.