Ancient Xfce desktop environment does not support 4K displays.
That is non-sense.
Settings | Appearance | Fonts … set the DPI to e.g. 138 (in case of a 32" 4K display)
Settings | Window Manager | Style … set theme to Default-hdpi or if you are on a smaller screen Default-xhdpi
That’s dom0.
Fedora templates: /etc/X11/Xresources … add or set Xft.dpi: 138 (or whatever matches your screen)
Debian templates: /etx/X11/Xresources/x11-common … same
If you have gnome-settings or another settings daemon installed in your templates (e.g. Fedora) you can either uninstall it and be done (my recommendation) or you need to install gnome-tweaks and run it in every single one of your qubes once to set the DPI there (annoying and same result as just uninstalling gnome-settings and rely on the Xft.dpi set earlier).
Everything is so small that it is hardly possible to use my laptop.
Yes, XFCE does not autodetect … you need to tell it.
And Qubes OS doesn’t attempt to do this because the setting is needed on each qube and depends on what you installed there (see above).
I tested Kubuntu and it was capable of running UHD displays with scaling option.
Scaling is just a GUI metaphor for DPI.
I write this on a 32" 4K display using XFCE and debian-minimal qubes. It works perfectly. Just follow the steps I outlined above.