You know how the latest version of OS X is 10.9.2? If you're running it, apparently you're exactly point-one versions away from "Retina" 4K support -- also known as "pixel doubling" -- in OS X. That's according to a beta tester who posted a screen capture of the new setting, per the just-released beta version of OS X 10.9.3 to Twitter last night.
KhaosT's monitor is a Dell UP2414Q, according to his screenshot. That makes it a Dell UltraSharp 24 Ultra HD monitor, according to Dell, with a native resolution of 3840 by 2160 pixels. That's 4K resolution, or what passes for 4K, since the marketing department won that particular spec-branding tug of war match ("3840K" sounds even kludgier than "1080p").
KhaosT says OS X 10.9.3 adds MST (Multi-Stream Transport) support -- a standard that allows you to daisy-chain multiple monitors -- and "the option for a user to select Retina resolution for 4K displays" running at 60Hz. That's another way of a referring to something sometimes loosely referred to as "pixel-doubling." Let's talk about that for a moment.
I've been running a 15-inch Retina MacBook Pro since Apple debuted its 2880 by 1880 (3K?) laptop screens in 2012, and that's basically what the configuration panel's looked like in OS X for me all along: an option to choose "Scaled," allowing you a range of selections from 1024 by 640 pixels up to 1920 by 1200, or "Best for display."
In my 15-inch rMBP's case, the latter's 1440 by 900, but my LCD's native resolution is 2880 by 1880 -- twice the functional resolution. If you run at "Best for display," you're basically getting 2880 x 1880's worth of gorgeous imagery within the bounds of 1440 by 900 of functional real estate, thus the ultra-sharp "Retina" look (whereby your eyes no longer discern individual pixels).
If you're running OS X on an older Mac Pro, or outputting to an external display from a MacBook laptop (Retina or no), recent versions through 10.9.2 only allow you to run at un-doubled resolutions of up to 4096 by 2160 pixels. 4096 by 2160 looks decent enough on a really, really big monitor, say 30-inches or more, but on anything smaller, the interface looks itty-bitty. And since earlier versions of OS X top out at sub-60Hz refresh rates, posing issues for motion-related interface features or applications, standard support for 60Hz is also a big deal.
In short, Apple looks to be rolling improved (and more versatile) 4K monitor support into OS X for pre-late-2013 Mac Pros as well as the MacBook Pro lineup (for those using the latter with external monitors).