Few specifics are offered in the release notes for build 14E7f, simply stating that 10.10.4 “improves the stability, compatibility, and security” of a Mac.
OS X Update Seed 10.10.4 is available to download through the Mac App Store for eligible developers, a code can also be redeemed from the Mac Developer website. The update is about 900MB and requires a reboot to install.
OS X Public Beta users currently do not have access to OS X 10.10.4 beta 1, though it’s certainly possible that will change, and presumably OS X 10.10.4 will be opened up to the larger public beta program at some point.
As usual with beta versions of OS X, you should always back up the Mac beforehand and consider installing on a separate partition if not a machine that is intended for beta testing.
Separately, for iOS developers, iOS 8.4 beta 1 is also available.
The most recent available public version of Mac system software remains OS X 10.10.3, though a small supplemental update to OS X 10.10.3 was also just released to address some potential issues with certain software configurations.