I'm using hidpi scaling on 13" 4K screen with Plasma/X11 just fine - that is, until an external screen that wants a different scaling factor is connected. This is still fine though; the only cases where I attach an external screen are presentations or other fullscreen content where the mismatched scale doesn't matter at all. However, if you want to generally work with multiple screens with various scaling ratios, you need to go Wayland indeed.