it depends on how big your tv is in your field of view, so a function of size and distance. and obviously how good your vision is.

My TV is also 4K but my amplifier which eats all the inputs can only do 1080p. 4k quality on that 65" is better, but not by that much that I'd throw 500+€ for a new amp since the current one works just fine and it fulfils all my needs on a TV/media set.