gtksourceview now offers version 4. It is still for gtk3. I expect a future gtk4 version will be named gtksourceview5.
The only part of core GNOME yet to switch to gtksourceview4 is sushi (as of the development 3.31 series).
It would be nice if gtksourceviewmm could switch too.
Reverse dependencies in Debian
cc/ @murrayc (in case you aren't watching this repo yet)