I have a non-retina imac 27 (1440p), external LG 27" 4K USB-C monitor and a macbook pro 13 with a real "retina", and use them all regularly.
For my eyes, scaling works fine with 4K - font rendering is significantly better than on 1440p imac.
13" screen on macbook pro is even better, and 5K 27" would be perfect, but that's a different price point. I'm quite happy with the improvement from 1440p => "4K with scaling" transition, and won't consider buying 1440p in future.
Scaled 4K may be not the best for high precision design work, but for development tasks / text reading that's an improvement, in my experience.