I have the Dell P2415Q and I consider the only true standalone retina display. 4k at larger than 24" gives you pixels too big for 2x (so text looks giant rather than retina-sharp) yet too small for 1x (unless you like to squint--and even so you don't get the doubled res).
That Dell exactly doubles the 1080p you'd get from a normal 24" display. And OS X can boost the virtual screen size larger--I like to go one size up, to 2304x1296.
BUT IT HAS TERRIBLE WAKE FROM SLEEP PROBLEMS with my Mac Pro. Sometimes I can only get it to turn on by force-shutting down the Mac, since I'm blind to do do anything else. And I received a number of DOA units. And the icing on the cake: Dell repeatedly refused to honor my warranty, saying that Dell displays above 23" from authorized resellers have no warranty. Only buy direct from Dell, they say. Tell that to Best Buy and Amazon! You may think I'm crazy, but I googled others with similar experiences. In truth, they DO have a warranty, but good luck getting it honored. Really: good luck.
So although I LOVE the color and sharpness of the Dell, I cannot truly recommend it. Keep waiting. Sorry! Retina is gorgeous on a big Mac screen, and the Mac Pro deserves it!