4K screens should work with Qubes, but I would not be surprised if there are performance problems, as Qubes OS only supports software rendering. Sadly, this is often considered a fallback path and not optimized. Firefox dropped its dedicated software renderer, and GTK’s Cairo renderer is deprecated. The replacement renderers wind up relying on software OpenGL, which is not efficient. I am not aware of any workarounds in the case of Firefox. GTK 4 software rendering performance can be improved by theming, but the themes that have good performance with software rendering are very flat and barebones. The increased rendering performance comes at the expense of ease of use for humans, especially non-technical users who dislike minimal UIs.