Missing GTK4 support

[6b6279]

stylepak seems to only copy over the gtk.css file for GTK3, which means themes that support both GTK3 and GTK4 are limited to GTK3. I tried changing the GTK_THEME_VER variable to 4.0 to create a seperate GTK4 theme, but 3. is partially hardcoded, so that the script so to say assumes that GTK_THEME_VER starts with 3.. Replacing the hardcoded 3. with 4. does do the trick. I couldn't get any GTK4 apps to use the generated GTK4 theme Flatpak, though.