More detail on the 10.9.3 beta’s 4K Retina support by Anand Lal Shimpi.
This is the real deal: this is desktop Retina, officially supported, on practical, affordable hardware. If you’re willing to run a beta OS, have a “Late 2013” Mac Pro or Late 2013 Retina MacBook Pro, and buy one of the supported 4K monitors, you can have desktop Retina today.
The ideal screen size is tricky, though. Using perfect 2X scaling like Retina iOS devices, which looks best and has the least GPU overhead, the “right” size for a 4K panel is 24 inches (like Dell’s UP2414Q), since it’s simulating a logical size of 1920×1080. If you currently use 24-inch or smaller monitors, that’s the way to go.
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. If it has the reduced-glare construction of modern iMacs, it’ll be tempting. (If it has the same ultra-reflective-glass construction as the current Thunderbolt Display, it definitely won’t be.)