The Heart of the Prince

Vegeta tries to leave for the fight, but Bulma convinces him to stay. She says that she's working on something that will help him. While she's preparing the device, Vegeta reminisces about Goku; everything from their first fight, to Goku's fight with Frieza, to their fight when he was Majin Vegeta. Bulma explains that after Goku was bombarded with the earth's flux waves on the Tuffle planet, he transformed into an Oozaru. Pan helped him become conscious of himself, and this made him transform again into the final Super Saiyan 4 stage. Since Vegeta is already conscious of himself as an Oozaru, he just needs to be bombarded with flux waves to become a Super Saiyan 4. He wasn't able to do it on the Tuffle planet because he wasn't a pure Saiyan then, since Baby was possessing him. To create these flux waves, Bulma has built the same machine that she built for Baby on the Tuffle planet.