While the toolkit certainly has an influence on all core apps that are part of GNOME, the shell is entirely independent of GTK. So, highly unlikely there will be a GNOME 4 any time soon.
Eh I bet there will be. Every Gnome/GTK version until 3.22 had the same version number and were released at the same time. The only reason that stopped is because GTK 3.22 was supposed to be "stable" and "the last GTK 3" version while Gnome still wanted to do updates.