The first 10.9.3 seed brought support for pixel-doubled retina resolution on 4k monitors.
This is a pre-release update to OS X Mavericks 10.9. Please refer to the seed note for more details and installation instructions
● Graphics Drivers
● Contacts & Calendar sync over USB in iTunes
Apple has also seeded a new build of iTunes 11.1.6.
Developers can download the new build from the Mac Dev Center linked below.