Personally I like 6's. Although I think the way you hunt really should dictate. I like to use 6's because I like to get close! It seems to me that at real close ranges 6's don't blow them up like 4's do. I have had to throw a few out after a close range hit with 4's. If you hunt like my brother does, 4's are probably better. He starts shooting almost as soon as he sees a squirrel and 4's seem to do a better job of knocking them down way out there when he does actually hit one. This is all opinion though...no data to back it up.
If shells were hard to come by, I'd use whatever is available and cheap. Preferably 4, 5, or 6 and I might even use 7's if that's all I could find.