4 cruisers! 
Oh, I got it into my head that he was sending a rather large force at you. He's probably just testing if you're active, so just send him a message saying "I'm active, please send the Cruisers elsewhere" and he'll move on. If not, then 5 Cruisers will do, or a few Battleships if you want to save up for 4/5 of them (less likely to lose anything then).
