"There's a form even beyond my current state right now. Let me show you a Super Saiyan's final stage, a Super Saiyan 5!" — Goku (Raising up the Playing Field! A New Power!)
Super Saiyan 5 is the strongest of all the Super Saiyan forms in the whole Dragon Ball universe, notably the only level beyond Super Saiyan 4 in Dragon Ball AF. It is the fifth and final form of Super Saiyan, the successor to all those that come before it. Son Goku was the first to achieve the form, through unknown methods. How to achieve the SSJ5 transformation is not known, though it has been shown that with Shenron's tremendous ki added on to Goku's, Goku was able to master the SSJ4 form, leading him to be a SSJ5, though he doesn't have proper control of it just yet.
Goku is the first to achieve it during his training in the Dragon Realm with Pikkon. This form's purpose is to unlock all the hidden potential inside the user, as well as to give rise to the addition of power, speed, endurance. It is noted that Goku has transformed into this form many times unintentionally during AF, as Pikkon declares that he would stand a chance against Goku in battle if he didn't keep on changing into this form. When Goku actually puts this form in use is when he confronts his evil son, Xicor. His SSJ4 transformation failed to do Xicor any harm which is why the mighty Saiyan takes it up a notch and tells them all there is a level even above Super Saiyan 4, which shocks the whole crowd, even Xicor. Goku powers up similarly to when he powers up for Buu and Babidi, and the tremendous power shakes the whole universe. Just turning into the form greatly damaged the Earth and the rest of the Galaxy, to which Xicor is blown away at its force. Fans also tend to claim that this form is equivalent in power to SSJ3 Gogeta. The answer to this is currently unknown, as the two warriors never chose to fuse again into the fused warrior against Xicor. Not only that but the SSJ5 varies drastically from its predecessor in both appearance and power. When SSJ4 Gohan, Vegeta, and Goku failed to do much harm to Xicor, the SSJ5 transformation was said to completely blow away Xicor. Even Xicor thought to himself that Goku's newfound power surpassed his own.
This form possesses a few setbacks such as the fact that its very power can destroy the whole universe if not properly controlled. This form has only been summoned in times of emergency, as Goku accidently used it to avoid a hit from Pikkon and Goku used it when nothing else worked against Xicor.
Son Gohan has also been seen using this transformation when he witnessed Pan's death at the hands of the evil tyrant who only wants to fight for pleasure, Ize. Gohan shook the Earth like Goku had and viciously started beating on Ize, whilst the other Z fighters gazed on with absolute surprise.
The transformation causes some stark changes in physical appearance. The trademark characteristic of the transformation is the user's hair: the hair of the Super Saiyan 4 state changes drastically, and grows down to or sometimes passes the user's waist, similarly like the SSJ3 form. But traces of the SSJ4 hair can be seen as well, forming bizarre hairstyle of silver hair. The brow ridge that is seen from the SSJ3 transformation is visible as well (though not as much as it is on the SSJ3). In this form, the Saiyan's hair color differs, the same goes for the tail and the body is covered in silver fur. A Saiyan in this form also possesses a shadow trim around the eyes and over the eyelids that varies in color, such as a dark bluish-gray or crimson. A similar aura like the SSJ4 transformation erupts in a SSJ5 as well. The combined looks of the two forms that come before this presumably marks this as the final form of a Super Saiyan.
In episode 55 of Dragon Ball GT, "The Heart of the Prince" , when Bulma tells Vegeta that she has been working on a way to get Vegeta to transform into a SSJ4, Bulma tells Vegeta that the use of her new machine may even make him go up to a SSJ5. This was probably only a joke, as the SSJ4 hasn't even been thought up yet and after Vegeta asks Bulma is she really believes the he can surpass Goku, she remains silence in embarrassment.
Though being depicted as a SSJ4, Vegeta never actually became one. When he faced Xicor in combat, he only was a SSJ4 at the time, as well as being a SSJ4 against Ize and other antagonists occurring after that.
The Super Saiyan 4 Transformation is the only other form besides SSJ3 to be a hybrid between Saiyan and Great Ape, though it is unknown whether there is a Silver Great Ape in DBAF.
The major antagonist of the first saga of DBAF, Xicor/Zaiko, is said to resemble a SSJ4 in a few ways. One being its eyelids, another is the same crazy silver hair. In some translations of the manga, it is said that Xicor himself is a SSJ4! Whether this is true or not, a fact that goes against this response is that the Western Supreme Kai/Lila (Xicor's mother) possesses a similar hairstyle with gray. He presumably inherited these traits from his mother, as he does not posses the fur or the tail that are require in this form.