Two main issues for me: I run Linux (4k support is simply _not_ there) and the input lag was very distracting. I tried a couple different 4k monitors, HP Z27 and LG 27UK850-W, and while the LG was slightly better, after a couple months I just couldn't bear it any longer.
I'm a full-time dev writing code mostly in Jetbrains IDEs. Hope this can spare somebody else the cost of trying 4k.