I agree that as part of the deposit a contract should be signed. In this case there was no contract and only 4 days had elapsed. The seller will have more opportunities to sell the puppy. I would refund the the money.
I had a different situation where someone had put a deposit on a puppy and then four weeks later she disappeared. Even though a contract had been signed and it was a non-refundable deposit, I felt I needed to return the deposit. She moved and did not cash my check. Eventually she called me to tell me what had happened but she would not take back her deposit money-- |