4 Tires or 2?

[holabird]

Need new tires on the front so went to the tire dealer and was going to get two new ones. He told me that since it was all wheel drive, I would have to get all 4 tires at once since they all need to be the same size. Made some sense, but they are not the same size now. The fronts are worn out and the backs still have some tread left. If I get two new ones, the new ones will have more tread than the ones I leave on, which is the same circumstance I have now. Is he blowing smoke or do I need to replace all 4? Thanks