> For example, with a 27" 4K display

4K pixels is not enough at 27" for Retina scaling.

Apple uses 5K panels in their 27" displays for this reason.

There are several very good 27" 5K monitors on the market now around $700 to $800. Not as cheap as the 4K monitors but you have to pay for the pixel density.

There are also driver boards that let you convert 27" 5K iMacs into external monitors. I don't recommend this lightly because it's not an easy mod but it's within reason for the motivated Hacker News audience.