GTK3 isn't deprecated (like GTK2 is), and there is no rush to port things to GTK4 yet. In terms of it being 'pre-alpha' (besides the docs, which in my experience before was always lagging behind), everything in the roadmap is basically done: https://wiki.gnome.org/Projects/GTK/Roadmap/GTK4
On the other hand, the changes in Gnome 40 were not as massive as the change from 2 to 3, and to me at least the change felt more incremental than disruptive (of course, the transition period is always a bit rough, some non-essential extensions for gnome-shell didn't work for me after the change, but eventually everything I needed was ported--I even dropped some extensions). I feel that saying that 'a lot of the work from 3 was thrown away' is an overstatement.
On the other hand, the changes in Gnome 40 were not as massive as the change from 2 to 3, and to me at least the change felt more incremental than disruptive (of course, the transition period is always a bit rough, some non-essential extensions for gnome-shell didn't work for me after the change, but eventually everything I needed was ported--I even dropped some extensions). I feel that saying that 'a lot of the work from 3 was thrown away' is an overstatement.