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.
Article Link: Apple Seeds Fourth OS X 10.10.4 Yosemite Beta to Developers, Public Beta Testers