After some basic maths* I found out why 1080p looks so much better (at least for me eyes).
4k@60MBPS: 1 pixel gets 7.23bits per second [ (60*10^6)/(3840*2160) bps/pixel ]
1080p@40MPBS: 1 pixel gets 19.29 per second [ (40*10^6)/(1920*1080) bps/pixel ]
That's a x2.66 times more information per...