My 6700k at work struggles to decode 4kp24 VP9 in software (in latest Chrome playing 4k YouTube content). Switch on hardware decoding, however, and my little GTX 960 does it without breaking out a sweat (windows reports approx 30% decode utilization).
Software playback of 4kp60 AV1 on 8 modern cores seems like a reasonable target to me, but it's kind of a moot point. Your average consumer watching 4k videos on YouTube probably doesn't have an 8 core CPU. I certainly don't, and I'm a hardcore tech / video nerd
I think YouTube will be using VP9 for 4kp60 VOD for quite some time, since there's lots of hardware decoders out there by now, and Google takes a very dogmatic position against HEVC