Hi Wendell,
would there be the chance to publish a HIDPI Guide (youtube or forums) for Linux?
I know it’s a mess… but anyhow, at least you could explain how YOU setup your 4K or WQHD Displays and Fonts on Plasma or Gnome?
That would be great! 
Thanks a lot and best regards from Germany
Daniel
Bumping this because while I dont have this problem right now at my current 1080p setup, I soon plan to upgrade it and maybe dont have to think for myself how to solve the problem in the future.
Running Fedora 35/36 with default Gnome on dual 27" 4k displays. Out of the box fonts are tiny and my glasses are not sufficient for productive work.
I found that simply enabling “Large text” from the Accessibility options is changing everything into a nicely workable environment for me.
For convenience I upped the default zoom level of Firefox to 120%.
Using this setup everyday since the pandemic started.
These simplistic steps don’t qualify as a HiDPI setup, but they’re easy and work for me. YMMV.
Cheers!
Gnome, Plasma, and Cinnamon all support scaling in the display settings UI.
There are fine-tuned controls if you want to tweak specific things (font scaling but not GUI element scaling, for instance), but “Enable display scaling” is really all that needs to be done for HiDPI on desktops that support it.
My primary workstation runs Cinnamon on a 4K display, and my HTPC drives a 4K TV on Plasma. I really don’t think about HiDPI at all other than remembering to set my theme and display scaling on reinstalls.
Ah - that reminds me. Yes, in Fedora the options for display scaling are 100%, 200%, 300%, 400%. These were useless to me. At the time (early 2020) I found online discussions suggesting that granular display scaling was a hidden feature which can be unlocked by tinkering with XML files. Alas, I could never get that to work. Purged that unfortunate experience from my memory …
Plasma has fractional scaling now and Gnome has fractional scaling on Wayland. Cinnamon does not officially support fractional scaling at all.
What is not yet well-supported anywhere is multi-monitor with independent scale factors.
Just double checked: Nope. Gnome on Wayland does not support fractional scaling in Fedora 35. Maybe I will confirm the same thing in Fedora 36 (current) later.
Dunno - maybe this is due to me using a NVidia GPU.
Because of artifacts and weird crashes I switched back to Gnome on X… Will all improve over time. Looking forward to that … until then Accessibility Options → Large Text it is 
Open a terminal, run:
gsettings set org.gnome.mutter experimental-features "['scale-monitor-framebuffer']"
Then re-open the display settings. Fedora might disable it, but Gnome absolutely does support this feature.
(Edited original command)
What i was wondeing:
On my Laptop 14" screen with WQHD i set 200% in plasma desktopfor the UI, but the font is still too tiny. So i have to set 120dpi font scaling additionally… but Plasma Desktop always reminds me, normally not to touch the font scaling… — Do you also have to set BOTH in your settings?
PS: watching Metro Exodus Let’s play in my spare time – sooo amazing graphics and story design! Like to start playing when back home…
