Apple today seeded the first build of OS X 10.9.4 to developers, approximately two weeks after releasing OS X 10.9.3 to the public. The new beta is available thorugh the Software Update mechanism in the Mac App Store should be listed soon in the Mac Dev Center.
The last Mavericks update, 10.9.3, added enhanced support for 4K displays and restored the ability to sync contacts and calendars between Mac and iOS devices over USB.
Article Link: Apple Seeds First OS X Mavericks 10.9.4 Beta to Developers