For a Saiyan to attain Super Saiyan 4, they must be able to transform into a Golden Great Ape (the Super Saiyan version of a Great Ape), and subsequently regain conscious control over their form. In Dragon Ball GT only, it is declared that the transformation cannot be accessed at will and requires exposure to Blutz Waves each time the user wishes to transform (Goku apparently being a special case as he is capable of transforming freely).
Link Download Char: Goku Xeno SS4 JUS
