I had the opposite experience.
I came from several years of 2560x1440 phones, at least 5 years , maybe 3-4 phones in there (I've always been a "screen real-estate guy" though, going back into CRTs, I really like density).
Anyway, I still haven't adjusted, fully. I can tell, because I use my Nexus 6 as a "small tablet" now, and the screen is dramatically sharper, still, every time I pick it up.
If OP doesn't decide to go to 4k, I'll probably be looking around, next time, in particular if the Pixel price and the OPx price get significantly closer.
My OP is a great device though, this is really one of the few (major anyway) gripes I have about it...