4K YouTube videos still not compatible with tvOS 14, picture-in-picture removed from website - 9to5Mac

The official YouTube support later confirmed on Twitter that users will indeed be able to play 4K videos on Apple TV with tvOS 14, but this feature is not yet enabled. Most likely the YouTube app will be updated to be fully compatible with tvOS 14, so users will have to wait a bit longer to get the 4K option. - Filipe Espósito, 9to5Mac.com [via/web:http://streaming-tv.us]