9 tails = Naruto dead & Kyuubi took over body
Jirairya was the one who taught naruto how to control the power of the kyuubi
First at all I have never been said that Sasuke was stronger than Naruto in four tails or in five tails. Orochimaru just said that you can't beat sasuke like this ( in the right translation). We have seen Bijuu losing against Akatsuki member and tha means doest that mean that the members of akatsuki are stronger than a bijuu itself?, short answer is not. A bijuu might have an bestial or demonic strenght and an amazing source of chakra, but a bijuu is a beast after all and they are not good at planning. Naruto went became a four tailed yokou and he lost control of himself. Basically he adpoted the personality of the Kyubi while he was a four tails youku. What we have seen so far from Akatsuki is mind over power. They use X strategy for each bijuu or jinkurichi and defeats them. They might have a strategy for Naruto too.
Now to anwser your question. If Naruto goes in five tails at the moment, it would be harder for Yamato to supress kyubi Chakra and recover Naruto. Yamato said that his Abilities are not equal to shodaime's abilities, so four tails youku form might be the represenation of the limit of kyubi's chakra that he can supress. Probably Kohona would have to decide between killing naruto or waiting till he calm himself.
alterno has given the best answer ever and deidara pritty much says the same thing when he talks about the sanbii being weaker than a jinchuriki because it lacks the intelegence to use power proply.
just because oro says naruto cannot win doesnt meen he would lose either it would just be a stalemate until naruto masters fourtails and has controle over all that power.
p.s shortening his life is no where neer saying he will die it just meens he loseses some time doing it.
basically sasuke gets owned?