Super Saiyan 4 is a Saiyan transformation present in the anime only sequel Dragon Ball GT, and that has never been mentioned in the Dragon Ball manga. The only way a Saiyan can attain Super Saiyan 4 is if he is already a Super Saiyan, can transform into a Golden Great Ape and regain conscious control over the form. This transformation is the ultimate combination of both the humanoid Super Saiyan and the Great Ape. After this form is attained once, it can be accessed at any point.
Usage and Power
The Super Saiyan 4 form draws the power of its users up to their absolute limit. The massive power and speed enhancements brought on by this form easily make it the strongest Saiyan form in the GT series. For example, while Goku's Super Saiyan 3 form was almost unable to even faze Baby Vegeta, as soon as Goku transformed into a Super Saiyan 4, their roles in power became polar opposites, with Baby Vegeta being almost unable to hurt Goku at all.
When Goku is given energy from Super Saiyan Gohan, Super Saiyan Goten, Super Saiyan Trunks, and Pan, his power further increases, gaining a red aura and becoming equal to a Blutz Wave amplified Golden Great Ape Baby.
The users of this transformation have powerful signature techniques. For example, Goku develops an attack he calls the 10x Kamehameha, as it is ten times (10x) stronger than the Super Kamehameha, while Vegeta, in his Super Saiyan 4 state, uses an attack he calls the Final Shine Attack, which is a more powerful version of his Final Flash attack (although, Vegeta is first seen using this attack against Super 17 with one hand while in his first Super Saiyan form; he first uses the more powerful two handed version in Super Saiyan 4), Gogeta uses a powerful combined attack named 100x Big Bang Kamehameha in video games, and Broly has an attack named Gigantic Destruction. Like the Super Saiyan God form, this form also seems to allow the user to absorb energy attacks. However, unlike Super Saiyan God, the Super Saiyan 4 form does not use up the user's power when absorbing energy attacks. This is shown when Goku absorbs the Revenge Death Ball Final attack by Great Ape Baby and turns it into a 10x Kamehameha.