As someone with firsthand experience in this field: it's not worth your time. Getting GTK4 to work in other desktops is a shitshow in the first place (it has horrible rendering issues on x11/Plasma configs), but all of the good stuff like gtk::Builder and connect-closures have been removed, making imperative development an enormous pain in the ass. If you still manage to make an app despite that mess, you'll be left with horrible font rendering issues that can only be fixed with a very specific Flatpak configuration (hope you didn't want to run your own app natively!) and by the time you've got it all figured out you'll probably be looking into Qt.

Please, just use GTK2 or GTK3 if you're planning on making non-GNOME apps. GTK4 is simply not finished yet.