Super Saiyan 4 (超サイヤ人フォー, Sūpā Saiya-jin Fō) is a Saiyan transformation present only in the anime sequel Dragon Ball GT, not mentioned in the 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 Golden Great Ape. After this form is attained once, it can be accessed at any point. It is also hinted that only pure-blooded Saiyans can achieve this final and ultimate Super Saiyan transformation, as Baby had full consciousness while as a Golden Great Ape, but he still did not transform. However, Baby was a parasite, possibly halting the final evolution. Another possible reason for Baby's inability to transform is that since it was Vegeta's body, Vegeta needed to be fully conscious, which he was not because of Baby's control over him. It is unknown if Gohan or any of the other Human/Saiyan hybrids could ever reach this form, as they too had tails at birth. Somehow, the Elder Kai already knew about the Super Saiyan 4 form and its abilities. It was unknown if the transformation is possible to use if the tail is removed until Vegeta transformed to SSJ4.
In this form, the Saiyan's hair color differs, the same goes for the tail and the body is covered in red fur. A Saiyan in this form also possesses a shadow trim around the eyes and over the eyelids that varies in color, such as crimson. The hair is lengthened, but not nearly as long as a Super Saiyan 3's. The aura in this form loses the arcs of electricity, instead becoming pure fire. However, in the case of the Fusion warrior Gogeta, the coloration of the aforementioned features seemingly switch,as Gogeta's hair became a bright shade of red whereas his fur became a dark shade of brown and the trim around his eyes were black instead of red. The massive power and speed enhancements brought on by this form easily make it the strongest Super Saiyan form in the series. For example, while Super Saiyan 3 Goku was almost unable to even phase 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. The users of this Super Saiyan transformation have signature techniques. For example, Goku develops an attack he calls "10x Kamehameha", as it is ten times (10x) stronger than the average Kamehameha Wave, 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 he is first seen using this attack against Super 17 with one hand.
Unlike the Super Saiyan 3's strain due to the utilization in ki, this form does not require as much energy consumption as Super Saiyan 3's, allowing the user to stay in the state at a much longer duration. It also appears that the form makes the individual larger in height and muscularity. In scenes with Goku standing next to others such as Gohan, Goten and Trunks, Goku appears much taller and bulkier than all of them, even though in his normal state, he is relatively the same size. A highly debated subject, many fans believe the Super Saiyan 4 form to be the true form of the Legendary Super Saiyan, first spoke of by Vegeta in the Namek Saga. Vegeta states in his monologue, that "the Super Saiyan could only maintain his maximum power in the transformed state". Also, Elder Supreme Kai appears to know of the Super Saiyan 4 form, cementing the thought that Super Saiyan 4 has existed before Goku attained it, most probably by a previous "Legendary Super Saiyan". The Super Saiyan 4 form even has enough power to break through a wish made with the Dragon Balls as it turned Kid Goku into an adult again.