Getting 1080p HiDPI to work on 4K TV

I've just gotten a new 2017 MacBook Pro 13", so this is not strictly hackintosh related, but I think it's an issue that others have had when connecting to high resolution displays anyway (plus you guys are more tinkerer-friendly).

With a Thunderbolt (USB C) to HDMI 2 cable (using active DisplayPort), I can get 3840 x 2160 at 60Hz no problem. But I want to be able to have it in 1080p HiDPI mode so I can actually read stuff from the couch. After searching, I found this site that generates a plist file to enable custom HiDPI resolutions:

https://comsysto.github.io/Display-Override-PropertyList-File-Parser-and-Generator-with-HiDPI-Support-For-Scaled-Resolutions/

I followed the steps in this guide to generate and copy the plist override file, and installed RDM to select resolutions. The new HiDPI resolutions show up in the menu (with lightning bolt):

https://imgur.com/AJkV0n1

But the highest 16:9 HiDPI resolution that works is 1600 x 900. If I click any scaled resolution higher than this, nothing happens. It just stays on the current resolution. As I said, unscaled 4k60 works fine, so it's not that the Iris 640 is not capable.

Any help much appreciated.