I have a 27 inch 2560x1440 monitor, with youtube 4k and test 4k media, I can tell the difference with 1080P vs 4K. Its a very big difference.
4K downsized to 1080P gives a great more detail, due to downsampling gives a higher detail, due to 1080P using 4 blocks with the same pixel, so 4k downsize, each 4 blocks are have a different pixel, its very noticeable. Chroma Sampling
Plus 60fps over 30fps YT is very noticeable, so thats another technology I want to see take off.
Also have a HDTV that is a normal 1080P HD tv, works great, yet i can tell the difference due to the low encoding rate on movies and the much higher on sports. Sports look absolutely amazing, none, none of the HD Prime movie channels are selling true blue ray quality tv. Comcast is ripping people off.
So do I want 4k? Hell, I want as fine pixels per inch as you can get, with the bandwidth to push it. We are no where a 50 inch tv running a high PPI, going to be a few decades away,which is a shame.