The GNOME Shell user interface and the Mutter compositing manager have been completely converted to use the GTK4 library and get rid of the hard dependency on GTK3. Also, the gnome-desktop-3.0 dependency has been replaced with gnome-desktop-4 and gnome-bg-4, and libnma with libnma4.
In general, GNOME remains tied to GTK3 for the time being, as not all applications and libraries have been ported to GTK4. For example, WebKit, libpeas, libibus, Evince document viewer, Totem video player, Eog image viewer, Boxes, Connections, Disks, Simple-scan, Photos, and System Monitor remain on GTK3.
Source: opennet.ru