Apple has launched OS X Mavericks 10.9.3 beta and it appears to have built-in support that enables all compatible 4K displays to be set at a “Retina” resolution, with an option for 60Hz output. This support could also indicate that Apple is working on their own 4K display.
The support was tested with both the Late 2013 Retina MacBook Pro and redesigned Mac Pro in conjunction with Dell’s UltraSharp 24 Ultra HD Monitor. Rumors also suggest that Apple will release a 4K resolution version of their Thunderbolt Display. The last release of the Thunderbolt Display was around two years ago.