YouTube will be available in 4K for Apple TV streamers soon.
Apple says users will be able to "Watch the latest YouTube videos in their full 4K glory."
Apple's change to support 4K YouTube videos is unclear. The 4K streaming box currently only supports 1080p even for UHD videos. The reason being is the lack VP9 support, YouTube's 4K codec. All of that is said to change with the release of tvOS 14. It's uncertain if Apple will start supporting VP9, another codec like AV1, or if YouTube will be adopting a new codec on their end. Either way, it's a welcomed change, especially for the pixel peepers comparing camera reviews and those who don't own a 4K monitor, but do own a 4K television and Apple TV 4K.
Other announcements at WWDC included a picture-in-picture mode for iPhones, immersive audio for AirPods, and that Apple will be making its own ARM-based chips, among many others. Look for the tvOS 14 update in the coming months.