I hope GTK3 will be still supported for a long time. GTK4 is more than a revolution IMO. It feels more like another toolkit which is using glib by accident as well.
From a user point of view, here is my personal shitlist for GTK4: - no icons on menu items - no classic file menu - no window positioning - only CSD - popup menus require compositing (https://gitlab.gnome.org/GNOME/gtk/-/issues/4815)
Fancy, modern interface experts might like this on their tiny exotic devices. I remember I had similar reservations from GTK2 to 3 but from 3 to 4 is even a way bigger step.
I think we will loose quite some features with GTK4 and need to think about the user interface aside from the technical aspects.
Disclaimer: the above is my personal opinion and does not claim to be correct or a good direction for the future.