Same here. I am also a Premium subscriber but not getting 4K on mobile even though it supports 4K resolution. Upto 1080p Full HD resolution is available.
Does your laptop have a 4K Display?
If not, then even if your are getting 4K option, the Display will only give an output of the supported resolution. You will be wasting data.
For example, my Laptop has a HD (720p) display. So when I try to play a 4K video, it will play without any issue. But I will be only able to see 720p quality. There will be no difference between 720p & 4K playback.
I don't have any laptop which supports 4K resolution with me.. I can't check that... What you can do is check for Stats for Nerds and check if your Viewport is supports 4K resolution...
Stats for Nerds: 5 clicks near Volume control option on Videoplayer.