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. Full info here.