...though you’d be foolish to buy a 24-inch 4K display, we can
only hope that Intel and Samsung’s ambitions can push down
prices on larger displays.
Reposting from my comment last year:
At 28", a 3840 x 2160 panel has a PPI of 157, which sits right between Retina and non-Retina densities. This means that on a Mac, you’ll have to use it one of two ways: Either at 1x, where the higher PPI means everything will be much smaller than it is on a normal monitor, or at 2x, where the lower PPI means everything will be much bigger than normal.
...
The best 4K monitor for Macs will be a 24”, which will have a PPI of 184, just about right for something sitting a bit further back from the viewer than a 220 PPI Retina Macbook Pro display.