Well I am using 4k 32 inch now, at 75 cm distance and I like it. Less eystrain than when keeping 27 at 60 cm.
It is absolutely fantastic. My eyes are around four feet from the screen and I don't get that blurred vision you get from extended near-work anymore. I could never go back.
It's super comfortable and as long as I'm careful about the enormous screen I can still use the table for projects when not on the computer.
As I sit at my desk now looking at my 27" 1440p (108ppi) display at a distance of 23" I really cannot fathom having double the pixel density. Could it be better? Sure! I chose 144Hz over density, but double the amount of pixels? No way in hell