I should add that the comments online I found about this bug were all related to the Mac Mini, I think. Makes me wonder if the bug only shows up when there is no microphone for Siri to use.
I have watched some 4k movies using the port connected to Apple TV and then looking at movies I bought that are in 4k, but not running them from my computer (because you can't download a 4k copy). So I go to the list of "purchased" movies and stream that way.
I also tried to look at some youtube stuff, but every time I actually played them and looked at the settings, it always showed 1080 - I finally found some info that said Safari no longer supports 4k! So for these things I go to Firefox now.
I'm always a bit leery about my impressions of 4k vs 1080 because my eyes limit me somewhat. For example, when I got the first ipad mini (no retina display) and then got the first retina version, if I was looking at a picture or movie, it was hard for me to see the difference even though all reviewers talked about how much better retina was. But put some text on the screen, and the difference was huge for me!
Speaking of the impression of things visually, I didn't get a color TV until 1988 - growing up we always had black and white TVs and even when color was common, I just didn't want to spend the money for color when the price difference was significant. But when I got color, it wasn't like I was wowed by the experience. It was certainly nice, but didn't make a huge impression on me. Maybe it's like people who like the old vinyl LP and think it sounds better - their minds filter out the hiss and pops, I think. For me, the hiss and pops always was very obvious (likewise with the hiss on cassettes) so when we got CDs and now digital files, that trumped everything else. Even if some old tech could be proven to be superior is some ways, it would probably be hard for me to tell and even if I could, it wouldn't be more important than getting rid of all that noise!
Just the time it takes to switch from a 1080p video to a 4k in youtube throws off my impression. It's funny, but some things I can remember for a long time, but many visual and audio things, unless the change is instant, I'm often not sure. To get a decent comparison, I have to pause a video and switch - still get a gap, but at least the image is identical.
I tried to find some 4k pictures instead of videos and they have some you can download, but the online versions I found are all much smaller. I prefer to download things as little as possible so I can't get a comparison that way.