If Apple were to beat the competition to the punch in bringing a 4K desktop screen to market, it would easily be a hit with tech enthusiasts. They would be fools to limit themselves to just Mac users.
Thunderbolt is good enough as there are pc motherboards with thunderbolt (gigabyte and asus).
The question is will there be a thunderbolt display with the new coating soon (next few month) and will there be a retina/4k display in the next few month as well?
In the sense of pixel doubling, an ATD would need 5120x2880 resolution, so more like 5K. This seems a bit far flung for the immediate future.
Maybe if they make a 21.5" retina ATD it could work. That would make their product line-up kind of lopsided with a better product in a lower size. This is why I think third-parties are probably going to beat Apple to putting out 4K desktop screens.