Although 4K Monitors are starting to become affordable, OS X hasn’t up until now supported them with the same sophistication it does a Retina Display. Even under Mavericks, the only readable resolution was 3840 x 2160, with no support for OS X’s Retina scaling options.
According to Anand Lal Shimpi of Anandtech, though, this has all changed, with the latest developer build of OS X 10.9.3 supporting the full range of Retina scaling modes for 4K monitors, allowing you to use those extra pixels to make things crisper instead of just smaller.
That’s good news… and probably a hint that whenever Apple releases the Retina iMac, it’ll boast a 3840 x 2160 display, and not the crazy 5120 x 2880 pixel display that would be called for if Apple just doubled the resolution of the 27-inch, as it has with other Retina Macs.
- Source Anandtech