I'm sure you're aware that there are a lot of charts and online calculators for viewing distance and angular resolution. I like this because it takes into account the visual acuity:
http://phrogz.net/tmp/ScreenDens2In.html
In my case with a 65'' screen at 1.5 m I can definitely see the difference between 4k and upscaled 1080p. But yeah, the content matters, I've been watching 4k transfers of old movies and the only benefit seems to be limited to the film grain.