4K displays might not be the pinnacle of desktop monitors for much longer. Dell is working on a new UltraSharp monitor with a truly ridiculous 5,120 x 2,880 display. That's precisely twice the resolution of the 27-inch iMac, meaning this is the type of display that Apple would have to use if it ever made an iMac with Retina display. And, as AnandTech points out, you could spin this display vertically and get two separate 2,560 x 2,880 desktops atop each other.
Dell's calling it a "5K" monitor, but that probably undersells just how many pixels are here. It has close to twice as many pixels as a 4K display — 14,745,600 versus 8,294,400. In terms of pixel density, it's just about equal to the 15-inch MacBook Pro with Retina display — it just happens to be a 27-inch monitor instead of a 15.4-inch one. There's no word yet on price, but it's expected that the Dell UltraSharp 27 Ultra HD 5K monitor will go on sale before the end of the year.