You have to bare in mind that AV1 decoding is not ready yet, it only just came out and Google only released those videos for everyone to test with. This is mostly to check how stressful the decoding is as well as design hardware decoders for it, as right now it's all done on the CPU. VP9, h.265/HEVC, h.264 and other codecs have been around long enough to have mature hardware decoders working fine for them.
We most likely will need new devices with proper AV1 support to see smooth stable playback, but that could be a couple of years.
EDIT: Ignore, I misread. AVC1 is h264 and should play fine. However unless I'm wrong I thought Youtube will only show content in 4K if it's using VP9, while h264 is for 1080p and lower.
You have to bare in mind that AV1 decoding is not ready yet, it only just came out and Google only released those videos for everyone to test with. This is mostly to check how stressful the decoding is as well as design hardware decoders for it, as right now it's all done on the CPU. VP9, h.265/HEVC, h.264 and other codecs have been around long enough to have mature hardware decoders working fine for them.
We most likely will need new devices with proper AV1 support to see smooth stable playback, but that could be a couple of years.
EDIT: Ignore, I misread. AVC1 is h264 and should play fine. However unless I'm wrong I thought Youtube will only show content in 4K if it's using VP9, while h264 is for 1080p and lower.