For bookmarks we use ~/.config/gtk-3.0/bookmarks to have compatibility with GTK3 apps. We also check the legacy file of GTK2 (why?).

I don't know about GTK4, but I think following GTK isn't a good idea after GTK3. Instead, I propose that we forget about GTK ≥ 4 and take a more self-consistent approach, by using a file of our own (perhaps ~/.local/share/libfm-qt/bookmarks), with ~/.config/gtk-3.0/bookmarks only for backward compatibility.