Apple today seeded the third beta of OS X 10.10.4 to developers, approximately two weeks after releasing the second OS X 10.10.4 beta and three weeks after releasing the first OS X 10.10.4 beta. OS X 10.10.4 has been in testing since mid-April, following the early April release of OS X 10.10.3, which included the new Photos for OS X app.
The beta, build 14E17e, can downloaded 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. 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.