Four Tails Kyuubi Form

During Team Yamato's mission to find sasuke,the group found and fought orochimaru.After the three tailed form was rendered ineffective, Naruto became furious and grew the four tail.During forming of four tail its chakara burns through his skin and form a black sphere that was mixed with Narutt's own blood.Naruto cannot achieve this form on his own, instead was forced upon his body by Kurama through the weakened seal, which allowed the four tail to grow.