Apple Seeds OS X 10.9.4 Beta 4
Apple today has seeded the fourth beta of OS X 10.9.4 to developers. This seed comes a week after the third one. At this point it's still unclear what this version adds.
Developers can update to beta 4 in the Mac App Store > Updates > Software Updates > OS X 10.9.4 beta 4. If you don't have OS X 10.9.4 yet you can download it for the first time in the Mac Developer Center.