Apple TV 4K has been launched for many years, but it has been unable to play YouTube with 4K resolution for many years. The reason behind it is that YouTube uses the VP9 video encoding system, and iOS or other Apple devices have not supported VP9 for many years.
However, with the support of VP9 in Safari and iOS, 4K YouTube can also be played including tvOS 14! However, when tvOS 14 was officially launched, users could not find the option of 4K YouTube. At that time, YouTube stated that it would fix the problem later.
Recently, some Apple TV 4K users have discovered that 4K options appear in the YouTube App. It is believed that YouTube has gradually lifted the restrictions on Apple TV broadcasting 4K YouTube.