The Qualcomm Snapdragon 805 in the Nexus 6 fully supports VP9 hardware acceleration. Though I'm not sure about the benefits of 4K video on a 6 inch screen.
I swear last I checked the 805 only supported hw accel. VP9 up to 1080 not 4K. The Qualcomm spec sheet says it supports up to 4K but reports out in the wild suggest that is "best case" and it doesn't really work beyond 1080.