Step 1: You tube app has not enabled 4K playing content yet, which is also played on 60fps. 

Step 2: Customer can play same content on laptop / PC using browser, we support all type of videos. 

Step 3: YouTube app will only allow you to play videos up to the screens max resolution. Which for the S8+ is 1440p (we got more width, but basically 1440p). So the YouTube app won't allow 4k to play. The phones hardware technically can play up to 4k 60fps, you can use the web browser to play YouTube from there (desktop version) but performance might suffer."