As of 10.9.3, Apple introduced some scaling modes for OS X that give you some more options:
http://www.anandtech.com/show/8023/apple-releases-osx-10-9-3-improved-4k-display
I don't know if they work with any 4K resolution monitor, nor whether they are "good" in terms of sharpness or what not.
At $1399, this is starting to get into the price range I'll consider one.
Today I'm using two Apple TBD's, and they are fine, but I wouldn't mind a non-glossy screen in my very bright loft, and also could give up a built in iSight to use an external USB cam.
I'd want some scaled resolution that would give me something comparable in screen real-estate for my non-media oriented coding work.