Apple TV support also isn’t universal, at least not yet. YouTube appears to be delivering the update remotely rather than tying it to an app release.
Still, this makes Apple’s device considerably more viable if you don’t want to pay to watch 4K videos. It also shows Apple loosening its policies on video formats. YouTube was previously limited to 1080p as Apple didn’t support the VP9 codec Google prefers for very high-resolution videos. Now, it’s just a question of whether or not app creators want 4K in the first place — and that’s highly likely as 4K TVs become commonplace.