Just a quick note: The affordable, and color accurate 24″ 4K display from Dell (UP2414Q) does work on recent Macs. Even my not so recent mid-2012 Retina MacBook Pro. However, on these, and even the later Retina MacBook Pro it only runs the full 4k resolution at 30 Hz. On the newer it should work with 60Hz, but this currently looks to be a Mac driver limitation.
Switching to HiDPI mode requires some hackery, as Mac does neither do this automatically according to the DPI that could be derived from the DDC EDID data, nor does it offer a UI option to do so, … :-/