there are no 4k phones from Samsung. playing 4k on a non 4k phone is a foolish thing to do so YouTube detects what's the max resolution your phone supports and enables videos upto that resolution.
If you have set your display to 1080p then the YouTube app will only show upto 1080p. Switch to 1440p under display settings, clear cache for the YouTube app and then run YouTube again, it should show you a 1440p option as well if it's available in the video. It shows 1440p for me.
