alt Hacker News> 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.
If your Mac goes bad it can be worthwile. My friend gave me his pre-Retina 27" iMac, part of the circa-2008 generation of Macs whose GPUs all failed.
I removed all the computing hardware but kept the Apple power supply, instead of using the cheapo one that came with the LCD driver board I bought. I was able to find the PWM specs for the panel, and installed a cheap PWM module with its own frequency & duty-cycle display to drive it and control brightness.
The result is my daily desktop monitor. Spent way too much time on it, but it works great!