I'm using a 4k iMac and it's amazing. Never thought it would be that great. My Cintiq 27" is nowhere near that screen and colour quality. However, I don't miss that at all when I paint with my 13" Cintiq Pro. The screen is top notch and the distance between the canvas and pen seems to be less than a 27" Cintiq. You can easily use it for professional photo editing, if you don't mind it being a Windows.
I also bought that new Cintiq Pro 13" last week and it looks better than the Intuos and 27" Cintiq. No idea how it is compared to the 16", though! Thought I'd try out the smaller version first and even though it is a bit smaller than I expected (being used to 27") I wouldn't buy a bigger one if I plan on taking it with me often! If you want to use it at home only I'd go for the 16" : )
If you want I can send you pictures of the two Cintiqs and Intuos S together but i don't have a 16" to compare it with.