The generation of extended needs to match the generation of the main display. So no, a 5 extended with a 4 main will not work. Nor will vice versa. 4-4 or 5-5 will. 

You can't do much of anything with a gen 1 4600 server/processor. Changing to a v2 will give you full function, no change in what extended will work with it or not.