All dahlias are tubers. If you bought them in 4" pots, they were likely started from seed (I don't know about the 6" pots), but will have formed a tuber now. Have you kept them in the same pot all summer, potted them up, or planted them out? If you kept them in a pot, you might be surprised at how large they will get if planted out. I'm not sure if they will winter over in your zone, but what I read says they probably will.

Here is a link that might be useful: Info that may be helpful