I play it on a 55 inch 4k. Games look amazing on it. I do have my TV's upscaling turned off. Games that run 1080p upscaled to 4k looks pretty good. But 900p looks kinda weird.
Looks okay, but nothing more. 1080 scaled up to 4k doesn't magically add detail.
What does help is that 4k TVs have better quality to begin with than 1080 sets, so you get better colour purity, brighter whites darker blacks.
I have a LGOLED and switch looks amazing on it especially zelda. Nintendos cartoon and clean animations really scale well and calibrate it just right and the colors just pop
It's good...but compared to games that render in HDR it's muddy as hell. I have a 4K HDR Sony Triluminous and games like Horizon, FFXV, Deus Ex and Uncharted 4 blow Zelda completely out of the water.