The core issue with 4k on Mac is that unlike Windows or Linux, Mac doesn't have usable 125%, 150% or 175% scaling modes. But at 1x font rendering on Macs sucks, and at 2x you basically have a fancy 1080p screen; it just doesn't fit as much as an unscaled 1440p screen.

This forces you into 5k screens, or tricks to make the OS do actually useful scaling.