External 4K display with 2012 MacBook Pro 15"
By installing SwitchResX I was able to choose 2560 x 1440 (60 Hz) which now gives me the screen real estate I need.



A really neat choice that doesn't give me as much screen real estate, but makes the screen into a retina 1080p monitor is the 1080p30 option. Everything now looks super crisp and I have the same screen real estate as on a full HD monitor.



A bit annoying that I need a $16,- third party application to fix this problem, but it's better than no solution at all.