Go ahead now and get the 4 month old a parvo shot. Even if it has had them before, they may not have taken. Frequently parvo is included in a combination shot, DLCPP or such. How soon? Right after your puppy digs in the wrong spot and unearths buried virus. At least with a 4 month old Pit, there is enough dog to have a better chance of surviving it. I have heard parvo will survive in the soil anywhere from 6 months to 12 years. Bleach and water is all that will kill it. For more on Parvo, see
Canine Parvovirus, What you should know about