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1.
This is one of the two Super Saiyan levels that do not appear in the original manga. The Super Saiyan 4 form can only be reached after transforming into a Golden Oozaru and then gaining conscious control over the form. In order to become an Oozaru the Saiyan tail must be fully extended. This form was first achieved by Goku when battling Bebi.

In this form the Saiyan's hair changes color (in Goku's case it turned black again, and Vegeta's orignal hair color has a tint of brown in it), the tail is present (due to the necessity of transforming into an Oozaru before taking this form) and the body and tail are covered in some shade of red fur. A Saiyan in this form also possesses a red shadow trim around the eyes and over the eye lids. The hair is lengthened and grows wild and untamed. This form, like Super Saiyan 3, can only be maintained for short periods of time. Only Elder Kai knew of Super Saiyan 4 and its abilities.

This form has the characteristics of a basic humanoid Saiyan and the "Ape"'s characteristics. This is why this form has natural color hair with Ape's fur and tail. This form harnesses the brute power of the Oozaru in the conscious hands of a Saiyan. Super Saiyan 4 is only possible for pure-blood Saiyans. This means Gohan, Trunks and any other half-breeds cannot reach this form. Vegeta, who was missing his tail, used a Blutz Wave emitting device made by Bulma (Bruits Waves in the Japanese version, a pun on 'brute' and 'fruits') to aid his first transformation.
"I'm amazed at how strong I've become." "At long last, I've ascended." - Goku and Vegeta on transforming into Super Saiyan 4
by mooses October 01, 2006