Instructions
Start your game and play through Dragon Universe mode as Goku until you get to the fight with Kid Buu.
Go to your ability tray from the in-game menu and equip "Spirit Bomb."
Enter battle with Kid Buu and defeat him using the Spirit Bomb attack without transforming into a Super Saiyan.
Travel to the islands east of the World Martial Arts Tournament. You will see a "???" appear on-screen, which initiates a short cut-scene. You will receive the "SSJ4" capsule after the scene is over.
Go to the in-game menu and equip the "SSJ4" capsule to your ability tray. You will now be able to transform into a an SS4 in future battles. Once in battle, charge your ki by pressing "X" and holding the directional pad away from your opponent. When your ki is full, press "Triangle," "Square" and "Circle" simultaneously to transform.