@AOKP doesn’t matter, I even tried 4k videos and only 720p shows up on both my Meizu mx4 pro and dads galaxy note 4.
I remember when he got his note 4 there was 1440p on compatible videos but after youtube updated their app 720p is the max, some people reported that after the most recent update of the app they got 1440p again on their note 4 but others didn’t and my dad also doesnt get it.
For some reason youtube has problems with high res videos on mobile.
Edit : My Ipad also is limited to 720p.
Actually it makes sense for them to limit it to 720p on mobile devices since there is no point of watching it on higher res on such small screen devices and it saves them A LOT OF BANDWITH which saves them money.