Bring up the stats for nerds menu in Youtube and it will tell you about the video stream. Most youtube apps have access to that but I couldn't tell you how with your TV but there should be a menu item somewhere in the Youtube app or when playing a video.
Looking at TV model on it's website it states it has VP9 support so you should be getting 4K Youtube though it may not list it as 4K, it might say 2160p instead.