We need 200 PPI or higher to achieve retina with large screen desktop monitor.
4K is destined for 21.5-inch iMac (205 PPI and 17" retina viewing distance) and 27-inch iMac and 27-inch Thunderbolt 2 Display will wait for 5120x2880 resolution panel (218 PPI and 16" retina viewing distance).
There is no defined retina standard. Even their original math made assumption on vision and viewing distance. Apple has been consistent in one area. They have used resolutions that were a factor of 2 over some lower resolution. I am curious what sizes they'll use. They have stuck with 21 and 27" for a few years at this point, but I don't expect to see 5120x2880 catch on in the near future. In either case it would look clean if implemented properly. Right now a 24" display is either 1920x1200 or 1920 x 1080. They both work, but it's a noticeable when compared against recent notebook displays.