GTK4 apps aren't meant to used outside of the platform for which they're built for. If the GTK4 app in question is built using libadwaita, it's meant to use only inside GNOME. If it's built using libgranite, it's meant to used only inside Pantheon (from elementaryOS).

Of course, you could use it in KDE or MATE or anywhere else but it would look completely alien with different themes and, potentially, different fonts and icons. You can't set your own theme without resorting to hacks.

If someone has come across a GTK4 app which isn't restricted to a specific desktop environment, let me know. Meanwhile, I won't use any GTK4 apps in existence.