I want to thank the original poster
I was able to get this working on a Mac mini 2012 and apple Thunderbolt Display 1440p monitor running high Sierra
I followed the guide installed switchresx, made a custom 3840x2160 resolution
after a reboot I did not see any hidpi options
so I did this terminal command
Want Retina-like sharpness on your standard definition monitor? Here's how to get it with HiDPI mode in Mac OS X, although there's a pretty big caveat.
www.techjunkie.com
after a reboot, I now see the 1080p HIDPI option and other hidpi resolutions in switchrex
text looks crisp not blurry and font size is 1080p large on the apple thunderbolt display