4k vs 5k Monitor as "Retina" on older MacPro
In case anyone comes across this question - here's what I made out of it. Given that the graphics card might get to its limits with 4k anyway, I chose to give the Eizo 3237 a try with the 4k resolution at 140 DPI. Running on native resolution is small but crisp, as expected. 

Using HiDPI modes and scaling up works really nice as long as you don't hit your nose onto the screen. 

Quality-wise this thing is an instant buy - although I might be biased as Eizo monitors have never failed to make me happy in the past (had a couple of good other ones as well - though I'd rank them a bit below in terms of perceived quality).