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 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, Old 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 Super Saiyan 4.