Molosser breeds do not even BEGIN to mature out until the age of 3. Until the age of 3 they're still considered puppies. So your dog is considered by molosser standards as just having outgrown puppyhood. At the age of 4 he is a very young adult who still has growing to do. Most molosser breeds to not reach their complete adult height & girth until the age of 5. (Posted as a former owner of 2 Neapolitan Mastiffs, 1 American Bandogge Mastiff & 1 English Mastiff)