I've never understood this. Why would SD look worse on a panel the same size at 3840x2160 compared to 1920x1080? Doesn't a group of 2x2 small pixels the same colour look identical to a single pixel at the lower resolution? SO even with the worse scaling method in the world it should look the same. Is it the gaps between the pixels that causes the problem?
It doesnt from a long distance away where SD would be acceptable. But there would never be 4 pixels the same colour from an upscale unless you uses a nearest pixel upscaling, which is what computers used in the 90's that made all the low res video look like a checkerboard. Modern upscaling is sooooo much better than that, and what is in a cheap TV today is better than any "high end" upscaler of a few years ago (except the bottom of the range stuff which is pretty bad)
When close up the edges can appear softer on a 4k panel than a 1080 panel. People equate hard edges to good picture quality which is why most upscalers add a crapload of edge enhancement which just makes things look like they are cardboard cutouts etc. The visible pixel structure makes the edges look sharper.
Yeah, I guess that's my point. If the upscaling is even just OK, then I don't see how a higher resolution display can look worse. Other than they are usually bigger and people get used to high resolution and forget how bad SD really looked?