While my home pc (old) chocks on 4k, my work - brand new i7 machine plays it smoothly with overall 10-15% usage..
However due to the extremely high compression end result tis tons of macroblocks and they rather large. Right now 1080p video looks like crap due to same reason - bumping the resolution to insane (only a dozen of displays in the world can show real 4K) doesn't help the PQ.