Originally posted by PiDicus Rex but the hard truth that Youtube doesn't want you to know, is that most peoples connections are only fast enough to show 720p, and only when they hold the 'phone landscape.
Depends on the country...
We are very lucky in Romania to have one of the best internet speed available worldwide (through cable my internet speed is 418mb for download and 472mb for upload). And I don't have the fastest connection available on my internet subscription plan.
Wirelessly I get 150mb for download with 170mb for upload on the 5Ghz wireless network. On my phone I really don't care if the Youtube video I'm watching is full hd or 4k. On a 27" Asus Pro Art monitor the differences between full hd and 4k are small, but on my 4k 60" smart tv I can clearly see the differences between full hd and 4k videos. So yes, if I'm at home, relaxing in my armchair, I tend to use the smart tv to browse on Youtube and if a video I'm interested in has 4k option, I go for it because depending on the content, it's a better experience than full hd.