OS X 10.9.0. In my conclusion I wrote: “4K display compatibility under OS X is still a bit like the wild west at this point”. Compatibility was pretty much only guaranteed with the ASUS/Sharp 4K displays if you cared about having a refresh rate higher than 30Hz. Even if you had the right monitor, the only really usable resolution was 3840 x 2160 – which ends up making text and UI elements a bit too small for some users.

For starters, the list of compatible displays in 10.9.3 extends beyond the ASUS/Sharp models. The 32″ and 24″ Dell 4K panels are now supported, and I’m guessing the < $1K 28" 4K monitors should be supported as well. You could drive any of those panels prior to 10.9.3 but not at 60Hz.

The display preference pane now includes support for scaled resolutions just like on the rMBPs.

The beauty of Apple's scaled approach is you get totally usable text/UI elements while still getting all of the benefits of having tons of screen real estate for working on images or videos.