Super Saiyan 4
Super Saiyan 4 is the ultimate form of a Saiyan. It is achieved when a Saiyan (or possibly Saiyan-human halfbreed, though this is unproven) achieves the Super Saiyan stage, and transforms into the Golden Oozaru form. If the Saiyan can gain control of themselves, they can transform into the Super Saiyan 4 stage.
This form is distinctly different from the other Super Saiyan forms in appearance- while this form does increase muscle mass, it does not have blond hair or green eyes. Instead, the Saiyan becomes covered in light reddish fur, and regains their tail (if they had lost it, that is). The power increase is substantial, but this form, like Super Saiyan 3, can only be maintained for a limited period of time.
Goku (as usual) was the first to become Super Saiyan 4, during his battle with Baby. Vegeta later accomplished this stage as well, cheating (in a fashion) by using Bulma's Brute Ray Amplifier invention to become a Golden Oozaru.
Comments
Some fans theorize that this is the true Super Saiyan stage, the one reached by the original Legendary Super Saiyan (who also transformed into a Golden Oozaru), and that the other Super Saiyan forms are aberrations created by the lack of a tail.
This is truly the highest Saiyan form- there is no official "Super Saiyan 5" level beyond this (though fans have made up a ton of unofficial ones).
