The issue mainly stems from the way macOS does UI scaling. To get UI elements the right physical size for a 27” 4K screen it would render to a 5120x2880 (i.e.

The issue mainly stems from the way macOS does UI scaling. To get UI elements the right physical size for a 27” 4K screen it would render to a 5120x2880 (i.e. 2560x1440@2x) framebuffer then do a non-integer scale back down to 4K. Sharper than 2560x1440 to be sure, but you can notice the blurring it introduces. Basically all large hi-DPI monitors for Macs are a compromise except the Pro Display, which is like $5000.