I am driven by what my eyeballs can perceive.
Monitors are used a bit further from the eye, compared to tablets/phones, so I am happy with a lower resolution that still reaches close to the "retina display" where I cannot see individual pixels.
So at 3-4 feet away, how big would the display need to be for 4K pixel density to approach retina density? I think 40 inches? That might work.
I'm also wondering how the introduction of affordable 4K displays will affect pricing on 30" 1600p displays, and 27" 1440p displays. I really really want to get a nice 30" 1600p display with the 16:10 aspect ratio, to match the 16:10 aspect ratio of the pair of 1920x1200 24" displays I already have, so I can do eyefinity with the center display providing a 'zoomed' effect.
With a 4K display, I'm not sure this would work without some distortion. But there is hope - anyone have a 4K display and can comment on how it handles switching to aspect ratios like 16:10, instead of its native 16:9? I'm very curious if it can do that by using letterboxing, without stretching? Meh probably I'll just end up running my 1200p displays at 16:9, as perhaps there is no way to predict from one 4K display to another, what kind of technique it will use to handle non-native resolutions and aspect ratios? Like what kind of scaler it will use etc.