The distance should be 2.5 times the diagonal measurement of the screen. I've not bothered to look at 4K TVs. When we bought our last one we had a problem finding one that would fit in the lounge (Victorian terrace with chimney intact).
Most seem to be sold as '4K ready', so it's going to be up to the Broadcasters to give you the benefit of 4k programming. How soon that will happen is anyones guess?