4k retina hidpi

I'm having an issue getting my display to output higher than 1080p. I can adjust the scaling on the display through system prefs to achieve a higher resolution, but I feel like I shouldn't have to do that... Maybe? I've got a 5k iMac that, by default, outputs 2560x1440, so I would expect a...

My monitor is AOC U2870VQE, that can up to 3840x2160 resolution. Cable is DisplayPort 1.2, I can get 4K@60Hz in Mac and Windows. GPU is AMD R9 290non-X(reference design). I can get 3840x2160, but the font is too small. So, how to enable HiDPI mode like retina macbook pro?