ssj4 is a new and improved type of super saiyan. goku was the first to pull off this awesome technique. later in db gt vegeta learns to pull it off and finally goku and vegeta do the fusion dance in ssj4 form creating ssj4 gogeta. they soon become a regular gogeta and ssj gogeta but ssj3 gogeta has not been accomplished yet.
gogeta
gogeta is the latest charecter in db gt he's supposubly the most powerful saiyan ever to exist!!
