While there are good 4K sources like UHD Bluray discs and a few decent streaming sources (but it's spotty and inconsistent), the vast majority of "4K" sources look pretty awful due to being wildly over-compressed. And the level of self-delusion from some of my friends – who are otherwise very smart – is near-emperor's-new-clothes level.
A recent example was the Super Bowl which was one of the few "special events" Comcast aired in 4K (only available on the latest version of Comcast's streaming boxes which few people have – and 4K is not available if you use DVR). I checked it out on a current-year higher-end 65" 4K TV (~$3k) which I adjusted myself to disable the myriad default settings claimed to "enhance" signals but often mangle them in unexpected ways.
Even set up properly on a good TV it looked terrible. The tell-tale DCT macro-blocking was rampant and in many shots the base 4K-ness of the resolution space seemed to highlight the 'peggishness' worse than the 1080p version (which I was cross-checking against out of now-morbid curiosity). The super bowl is widely claimed to be the best quality 4K live broadcast. $250,000 lenses on the best cameras sent through the best signal paths managed by hundreds of engineers across a small city of the world's best production trucks. All stomped into a peggish mess so that cable and satellite companies can shove hundreds of channels of infomercials and decades-old Murder She Wrote episodes down the pipe simultaneously. I guess they'll keep doing it until enough people demand better or someone offers an alternative. Afterward I searched online and it appears the only widely accessible way to view the SB in decent 4K was if you live in one of the few cities that currently have broadcast TV stations airing true 4K and you set up an over-the-air antenna. While it's still compressed, due to FCC specifications for signals actually broadcast OTA, it's pretty mild and apparently looks sensational compared the what most people saw at home on their "awesome" new 4K TVs.