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As for my advice, I would take the same steps to introducing puppies as adult dogs. I got Sylvia (see below) in June and she was about 4 months old and then in August we adopted Harley a one year old (also below). Their ages were close, but after a few days they were pals. Don’t forget that the new puppy and the four month old and going to need some time to adjust. I suspect the four month old is still adjusting to you home now. In my opinion, I think it is a good time to get another puppy so neither pup has really established dominance. My advice is to have a lot of patience, you're gonna need it!