Over the earlier GTK4 development releases, GTK+ 3.90.0 now allows a custom renderer to be used by the inspector with the GTK_INSPECTOR_RENDERER environment variable. There is also a shift of moving container functionality to GtkWidget. But beyond those two changes, the rest of the work is focused on bug fixes. Details via this Git commit.
Among the changes so far reached by GTK+ 3.90 include merging the GTK Scene Kit (GSK), adding an optional Vulkan renderer, removing deprecated APIs, OpenGL improvements, frame-by-frame record/replay in the inspector, Windows HiDPI support, Wayland improvements, and much more. GTK 4.0 so far is shaping up quite nicely under their tool-kit plans.