So tell us what the difference is since most can't see it
Complicated answer, for sure.
Just like you can see the difference viewing the same HD content on two different brands of phones, there can also be a similar difference, of course, on different brands of TV's. There can even be a marked difference with the same brand using different technologies (LED vs OLED, for example).
Now, once you get into HD, and very much so with 4k, size also matters. But when I talk about size, it's not so much the size, but viewing distance. Sony, for example, when suggesting viewing distance for one of their 4k TV's will suggest a viewing distance that's about 1/4 of the viewing distance suggested for one of their HD TV's.
So, given the same content on the same TV (removing the variables I first mentioned here), if you are not close enough given the size of your TV, you will likely NOT see much of a difference between HD and 4k. So, if you have a 4k TV and are sitting too far away from it you are wasting your TV.
On my 65" LG OLED (reputed to have one of the best 4k pictures), if I'm more than 6' away from it there isn't a lot of difference in picture quality from an upscaled HD (to 4k) source vs the same source in 4k. There's a difference for sure, but at 10 or 12 feet away the difference is small. At 5' the difference is pronounced.
So, when folks talk about the difference or lack there of when viewing 4k content, who knows what they are actually doing and doing it on to formulate that opinion.