Apple has released the second developer beta, and first AppleSeed Beta of OS X 10.10.4
Apple notes that OS X 10.10.4 "improves the stability, compatibility, and security of your Mac". Apart from that, there are no specifics about exactly what is to be found in this release.
Developers can grab this build from the Mac Developer Center, or the updates section of the Mac App Store if they have the latest beta installed. ApplSeed testers can also grab the update from the Mac App Store.
The build of this version of OS X 10.10.4 is 14E11f. Apple released the first beta build to developers earlier this month.