Native or "true" 4K streaming can only be done if youtube is broadcasting that video in 4K or if that video was uploaded in perfect 4K quality. Typically uploading videos to youtube get degraded in quality. If you want to see genuine 4K, I suggest you purchases a 4K movie from Best Buy. I have a 4K HDR tv and the Xbox One S, I am upscaling my bluray 1080p Game of Thrones to 4K and it looks spectacular. Hope this helps...
Also make sure all settings for 4K are on. Your television might be a 4K tv that is still set to only show 1080p picture