Added warning message in Face4 as suggested by @icannotfly in #3663.

src/core/Face4.js

@@ -4,6 +4,8 @@
 
 THREE.Face4 = function ( a, b, c, d, normal, color, materialIndex ) {
 
+	console.warn( 'THREE.Face4 has been removed. A THREE.Face3 will be created instead.')
+
 	return new THREE.Face3( a, b, c, normal, color, materialIndex );
 
 };