This is the fourth transformation of a super saiyan, and the successor of super saiyan 3. It was first used against Baby in Dragon Ball GT.
Goku first transformed into a great ape by using earth as a moon. He transformed into a golden great ape instead of the original form. He went on a rampage but later controlled himself thus turning into a Super Saiyan 4.
The people confirmed to be able to turn into this form are: Goku, Vegeta, Gogeta, Purika and Broly. Other possible people could be Vegito, Gohan, Goten, Trunks, Future Trunks, Kara.