my 24" external 4K display no scaling works flawless with my 15" 4K 150% scaling notebook. (fedora 35/wayland)
This guy knows what's up.
If this doesn't work nicely for you, you're probably running X11 rather than Wayland.
Wayland still doesn't solve this problem because many apps (GTK) become blurry as a result of being scaled poorly, and moving them between displays results in weird behavior where they retain their original scale value rather than adapt to the new display's scale. It's not a good experience.