In particular, the NEC PA272W BK-SV has a $400 rebate. It’s a 2560 X 1440 display with the NEC SpectraView II softare and hardware calibration unit included. It an be used in portrait and landscape mode, making it an excellent primary or secondary display.
I still think that a conventional pixel ratio (non retina) is better for editing images at any detail level (or a large 4K display such as the 32-inch PA322UHD, to keep pixel density reasonable for detail editing). But even if one is planning on a 4K display, a 2nd display is mandatory for my work for palettes and extra stuff so that the main screen remains uncluttered.