sums it up. and, I sort of agree too, although I generally believe it's because of the capture camera and the compression software. When I look at videos recorded by 4K cameras but at 1080p, it does look better than 720p. When I look at 1080p videos recorded by 1080p cameras, I rarely see a difference with the 720p version.
Now for the screen size, it depends on the room size, but, when I go to my friend with a 100 inches projector (as in its the final picture size on the wall), that's definitely appreciable :p
Also, 1080p stream already makes me hit stupid ISP data caps, 4K, well, uhm, not sure I've got the bandwidth anyway :)
See also: http://www.youtube.com/playlist?list=PL5BF9E09ECEC8F88F