Actually, you can.
It all depends on what the question actually is. Yes, I know this sounds stupid but think about for a second. You have run out of space and want to add more capacity, yes ? You are asking a question that sounds like "can I add more disks to my current pool to extend the size as I have run out of space". Pirateghost is correct, you can't, however, there is a way around this. Unfortunately it does mean purchasing four new hard drives (of greater capacity than what you have now) and, one by one, replace each disk with one of your new ones, letting FreeNAS handle the resilvering (pool rebuild) for you. I have done this myself and it works fine. I replaced my original pool consisting of 4X1TB with 4X2TB drives, then kept the 4X1TB as a backup pool. Search around the forum and lots of other folks have done it as well.