Wow this one blew up fast. Sad I missed it earlier.
Being agnostic simply means you don't know. You lack knowledge. I do agree that one could use the word to describe the nature of one's theism or atheism. But it can exist on its own to denote a 5th stance skipped by the OP's list of 4 options.
Is there a god? Can you prove it?:
There is! I can!
There is! I can't
There isn't! I can!
There isn't! I can't
#5 Don't know / Don't care