From what I've heard surrounding this, GTK3 to GTK4 isn't as big of a jump as GTK2 to GTK3 was. The GTK3 port was finished first because there was already work in place for that, but we can expect a GTK4 port to be faster. That said, I haven't seen many apps that aren't specifically GNOME apps start using GTK4 in the first place, and as such I'm currently not using any GTK4 applications. I expect it to take a while before more things move to GTK4.