4k is on the edge for this hardware ... some codecs might work well.
MALI is unrelated to video decoding. It is a 3D engine. What you want is:
http://linux-sunxi.org/Cedrus and
http://linux-sunxi.org/CedarX
Bringing this support to Chromium seems like a hard task since so people try to add plugins which play videos in external (HW accelerated) players. In case you want to watch Youtube videos in full HD, this can be done.