A quick update:

I have now tried using both Arch Linux and the latest Ubuntu installer in addition to Qubes OS, but I still have not been able to get a 4K resolution on a Librem13 laptop (ordered in January 2018).

I tried to use cvt -r 3840 2160 in order to produce a modeline for my system, and while I can see the resolution in the XFCE Display settings (after having configured xorg), I can't switch to it.