I guess people (like me -- or maybe just me) are after the macOS-like way of 150% scaling on a 4k display, which uses a 5k buffer with 200% scaling, then scales down the entire image to fit the screen.

This results in a 2560×1440 effective resolution with a bit of blurriness that is acceptable, compared to "native 200%" scaling that makes the controls a bit too large to my liking, or "native 150%" scaling that is hidden behind some "trust me, you don't want to see this" language/button in GNOME.

Things do seem to be slower when it is switched on, which is not the case with the 200% and 100% settings.

The size mentioned, 4k@24” is properly sized for 200% with perfect quality. So no shenanigans needed.

This size is very difficult to find these days, presumably because people don’t understand this. Or hardware co. mgmt didn’t.

TL;DR A 4k monitor should be the same size as HD, with 200% scaling. 22” aka 56cm is about right.