Perhaps Kabuto possesed the body of the 4 year old boy using Orochimaru's technique thus making him the perfect spy. Also if this is true then perhaps Kabuto had some conection with Tsunade to learn some of her medical techniques. Maybe Orochi has known him since he was still with the leafs giving him some sort of connection with Kakashi. Maybe he was even the 4ths 3rd student, the first being Kakashi and the second being the cigarette guy (i forgot his name).