Knowing the recommended seating distances vs. screen size for 1080, I asked a salesman at a store how close I needed to sit to properly resolve 4k. He looked at me like I was crazy for asking, and of course had no idea of the answer. The answer is that for a 65 inch TV, you need to be at just over 8 feet to even begin to make it distinguishable from 1080p. 6 feet is more ideal. On one hand, we just shrunk the required size for a theater, though no one will be sitting that close to the TV in their living room. On the other hand, you still need a large room for a proper sound stage. And now you run into problems setting up that sound stage based on how close you need to be to the front/center channel. It is a problem that will probably take the audio side some time to solve.
At 15", I actually sit 6" inside the THX recommended viewing distance for HD content. My vision is 20/15 or so, and I have no issues at all with resolution at that distance. But, the 37-38 degree viewing angle is as far as wide as is comfortable (for me). For others, it may be, and probably is, too wide of an angle. That distance would would on the far side of the wheelhouse for UD/4k. I would no doubt experience some visual gains from it. Unfortunately, the viewing angle prohibits me (and probably everyone else) from taking any further advantage of the increased resolution. Ideal eyeball distance from a 4k source on a 140" screen is 13'8" which is a 40.4 degree viewing angle. As weird as it sounds, with higher resolution, maybe a switch back to 4:3 is warranted???
And then there is the question of content which kills the whole idea outright at the present time.