Anand Lal Shimpi, writing for AnandTech:
In my Mac Pro review I lamented the state of 4K display support under 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 × 2160 – which ends up making text and UI elements a bit too small for some users. Absent were the wonderful scaling resolutions that Apple introduced with its MacBook Pro with Retina Display. Well it looks like that won’t be the case for long, last night I got reports (thanks Mike!) that the latest developer build of OS X 10.9.3 includes expanded support for 4K displays, 4K/60Hz support for rMBPs and scaled resolutions below 4K.
I’m inclined to agree with Marco in his post on the 4K monitors:
If you use a 27- or 30-inch monitor, though, you’re accustomed to a resolution of 2560×1440 or 2560×1600. These scaling modes will simulate that on 4K, just like Retina MacBook Pros simulate higher resolutions, but the larger you go above 24 inches in size, the worse it’ll look. The biggest size at which a simulated 2560×1440 could look reasonable is probably 27 inches — I certainly wouldn’t buy a 4K monitor for Retina use larger than that.
Personally, since it’s probably coming within a few months, I’ll wait to see how Apple’s theoretical 4K 27-inch Thunderbolt 2 Display turns out before buying anything.