Retina on iMac pros are 5k
But, like all prebuilts, they ruin a perfectly good monitor by paring it with crap GPU, and a mobile GPU at that (m290x with option for m295x), I'd say apple might make a killing if they released the retina display on its own.
Except that Apple uses some trickery to actually get 5k on the monitor. When put on windows, it can only run at 4k. Still a nice display though.
https://www.youtube.com/watch?v=tgTMxB-ffjM&t=222