Apple today seeded the first beta of OS X 10.10.4 to developers, just over a week after releasing OS X 10.10.3 with the Photos for OS X app to the public.The new beta can be downloaded through the Mac Developer Center.
It is not clear what's new in the beta, but it may include new features like OS X 10.10.3, or it may be a smaller update focused on bug fixes like OS X 10.10.1 and OS X 10.10.2, both of which introduced several fixes to improve performance.
Apple Seeds First OS X 10.10.4 Yosemite Beta to Developers