The new Apple TV 4K supports YouTube 4K video playback running at 60 frames per second when using the YouTube app. This is an improvement from the 30 fps cap before.
YouTube recently expanded its 4K support for Apple TV in October, but playback was largely limited to just 30 fps. The new Apple TV device has a more robust processor that can handle the 60 fps for 4K video playback. It’s only available on the newer device as the old Apple TV 4K does not have this option.
4K video playback at 60 frames per second is only available to devices with greater processing power. YouTube determines this using a codec. Aside from the faster processor the new Apple TV 4K has a redesigned Siri remote that comes in with the purchase. The Siri remote can be bought as a standalone product and is compatible with older Apple TV boxes.