New Build of OS X Mavericks 10.9.3 for Testing

Apple has released a new build of OS X Mavericks 10.9.3 to test for developers, The build number is 13D38.

Apple continues to ask developers to focus on Graphics Drivers, Audio, Mail, Contacts and Calendar sync over USB in iTunes, and Safari. As was discovered with the first beta, 10.9.3 adds new support for 4K displays, offering "Retina" resolutions that improve readability along with support for 60Hz output from the Retina MacBook Pro.

you can get it  through the Software Update in the Mac App Store as well as the Mac Dev Center.