Before this update, users with Macs connected to a 4K display would simply experience their content on a larger scale with window elements spread out. Macs on 4K displays essentially were given ~4000 lines of room to accomplish their tasks. Prior to this 10.9.3 beta, users have solely been able to run their Macs at a Retina/pixel-doubled resolution using unofficial hacks.
Khaos Tian was also able to confirm OS X 10.9.3 brings support for 4K@60Hz on the new Mac Pro and the Retina Display MacBook Pro (late 2013).
Apple has yet to release a 4K monitor of its own. Hopefully these improvements bring us a little closer to that happening.
Read More [KhaosT]
OS X 10.9.3 Beta
OS X 10.9.2