Is it worth getting a 4k monitor for a late-2012 13" MacBook Pro with Retina display?
Solution 1:
What's officially "supported" and what's possible don't match. I managed to get 4K out of it at 30Hz.
I took a screenshot as proof:
Just a normal mini-displayport<->displayport cable was used.
More details in my answer here: https://apple.stackexchange.com/a/147765/39878
or on this blog post: http://www.mattburns.co.uk/blog/2014/09/30/running-the-4k-aoc-u2868pqu-and-intel-hd4000-graphics/
2560x1440 works ok but because it isn't HiDPI it still feels like a step backwards. I flip between running 1080p (HiDPI) and full 4k (with the text being tiny).