Nice. Maybe in a handful of years there'll be applications besides GNOME itself that use Gtk4 and maybe the documentation will even exist. But right now the vast majority of applications use Gtk3 and Gtk3 has been frozen broken for a decade. Right now you literally cannot paste a file path into a file->open dialog without invoking some arcane key sequence to bring up the "path bar" first. If you just paste into a newly opened file->open dialog it litterally errors out. That basic functionality does not exist, let alone a luxury like thumbnails.

I've talked to the Gtk devs about this and they say that 3 is will not be fixed, ever. They won't even accept patches because gtk/gtk/gtkfilechooserwidget.c is so cursed.