Vegeta SS4 (left) and Goku SS4 (right)

Super Saiyan 4 is a transformation achieved through a Saiyan transforming into a Golden Great Ape (the version of ape accesed when one becomes a Super Saiyan) and regaining consienceness. Only three Saiyans have achieved this form (if one counts Gogeta as a seperate Saiyan ). Goku was the first to achieve it, then Vegeta (only by using a machine), then they fuse together to create Gogeta. This form is not seen nor mentioned in the manga.

Fans have deduced that Super Saiyan 4 has a power upgrade of 500X normal power.