If you're not using a discrete GPU, that addition very possible will open up some more HiDPI resolutions for you. With a modest RX-460 (recognized in 10.13+) I get a variety of formats starting from a 2560x1440 layout over 3840x2160 pixels, and incrementally more spacious, all still over 3840x2160 on the wire, all with excellent HiDPI rendering. My intel graphics iGPU delivers just the limitations you described.