Orochimaru can be beaten by 4 Tails, i'm not sure why you can't understand this. Naruto nearly has perpetual Chakra in this form, while Orochimaru will eventually weaken. Naruto will keep coming until he kills Orochimaru in that form, and Orochimaru can only hold his defense for so long. You merely saw an example of what would happen in Extremely Best case scenario for Orochimaru, which included Yamato being there.