Earlier quoted context omitted.
2560x1440 on a 27 inch at a reasonable distance is pretty darn close to optimal IMO, so 4k, to me, is for 34" monitors (but 27 inch I feel is optimal on 60-75 inch desks, which is what I usually work with, so 4k rarely matters). I'm with you on accurately calibrated monitors though! God most of them suck out of the box.
If you don’t see any (spectacular!) difference between 4K & 1440p you need to have your eyesight checked. I’m not being sarcastic. The last time there was a thread like that on HN a bunch of people figured out they need glasses. I have a 4K @ 24in monitor (180ppi) and a 267 ppi netbook and when I switch between them the 4K starts looking like a blurry mess!
Don't get me wrong, I can see a difference, but not nearly as night and day, especially when it comes at the cost of other features (eg refresh rate... which isn't the end of the world for coding so if it's the only thing you do on the monitor it could be worse... otherwise ouch my eyes.)