I’ve branched GtkSourceView for 4.6 (gtksourceview-4-6) which you should be using instead of master for your application’s Nightly Flatpak builds. I will land the GTK 4 port on master early next week. A message to gnome-announce-list has been sent and will hopefully make it into distribution packagers inbox shortly.
Long story short is that the 4.6 series will be our long-term (and last) series for GTK 3 applications. I expect this to be maintained for many years. Master will become the beginning of our transition to GTK 4 and the place we land lots of upstream features for Next.0.