Oddly, my 4k TV seems to give a remarkably good picture on normal, 720p, 1080p and 4k material. Much better than its predecessor, which was supposedly full HD but which actually had a slightly-less-than-HD panel (I think it may have been 1680x1050), meaning that nothing lined up, horizontal or vertical, leading to horribly soft pictures.
Before I discovered that Marketing's idea of "Full HD" didn't necessarily mean 1920x1080, I spent endless frustrating hours trying to get a pixel-perfect display out of a Raspberry Pi (back when they were new). It was a lost cause.