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.
Article Link: Apple Seeds Fifth OS X 10.10.4 Yosemite Beta to Developers and Public Beta Testers