If the rumours are true that 4k streaming will require 15Mbit/sec then it's going to be pretty low quality even if it's HEVC. The current 1080p movies/shows on iTunes are also very low quality (5 Mbps) and it shows because the picture is washed out and many details are lost. 720p iTunes is a lot better in this regard as it's 4 Mbps and looks better than 1080p.