README: Add GTK 4 to the list of supported toolkits

Author: jnsh

README.md

@@ -29,6 +29,7 @@ This fork aims to keep the theme updated with new toolkit and desktop environmen
 Arc comes with themes for the following:
 * GTK 2
 * GTK 3 >=3.18
+* GTK 4
 * GNOME Shell >=3.18
 * Cinnamon >=2.8
 * Unity