KN4 is the abbreviation of Kyuubi-Naruto-4 from the Naruto anime/manga. KN4 is the form in which Naruto loses it and starts a rampage. It is so far the greatest number of tails he's able to get when releasing the Kyuubi's Yang chakra.
Naruto's demon fox's cloak form with four tails is called KN4.
by Ittouchy January 28, 2008