I've tried the Cairo-based Clock demo, and adjusted my code to reflect needed changes to port to gtkmm4. Now, I'm currently missing mouse activities.
In addition to what Andrew Potter suggested, if you want to track all mouse movements, use Gtk::EventControllerMotion.
Just about everything that was done in event signal handlers in
gtkmm3 is now done with the many subclasses of
Gtk::EventController.
I've read the gtkmm4 changes section of the tutorial (https://developer-old.gnome.org/gtkmm-tutorial/unstable/gtkmm-tutorial.html#changes-gtkmm4) but content is more of a note than actual information for beginner porters like me.
I don't think anyone will ever take the time to write a real migration guide from gtkmm3 to gtkmm4.
The documentation at developer-old.gnome.org is not updated any more. If you want to see the latest version of the gtkmm4 tutorial, look at https://gnome.pages.gitlab.gnome.org/gtkmm-documentation.