Retina is determined by view distance and visual acuity. You can get retina on any 4k tv right now at several meters of view distance (hell, you can get "retina" on old 1080p tvs when you are sufficiently far enough away), but if you stood right in front of it it would be obviously pixilated anywhere from 30"+ resolution.
Same applies to desktop monitors and phones. The reason phone screens pushed high DPI first was because you were much closer to the screen to make it fill your view.
Same applies to desktop monitors and phones. The reason phone screens pushed high DPI first was because you were much closer to the screen to make it fill your view.