I use a 1080p monitor and what I’ve noticed is that once creators start uploading 4k content the 1080p version that I watch on fullscreen has more artifacting than when they only uploaded in 1080p.
Did you notice that as well?
Watching in 1440p on a 1080p monitor results in a much better image, to the detriment of theoretically less sharper image and a lot higher CPU usage.
It may be worth right-clicking the video and choosing “Stats for Nerds” this will show you the video codec being used. For me 1080p is typically VP9 while 4k is usually AV1. Since AV1 is a newer codec it is quite likely that you don’t have hardware decoding support.