My desktop monitor is a 54" 4K TV that I sit about 3' from. It's somewhat difficult for me to pick out individual pixels even when I lean in. My living room TV is 70" 4K, but I sit 15' away from it. There's no way I could tell the difference in 4K and 1080 from pixel density alone. I can however tell the difference between 4K and 1080 streams because of how shitty low bitrates look. 4K streams crush all of the dark colors and leave you with these nasty banding effects that I don't see as often on lower resolution streams.