Végéta s'en mêle

Vegeta tries to leave for the fight to join Goku and the others, 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 in the past; everything from their first fight when he was once evil (though he was still good in the present), to Goku's fight with Frieza when he first transformed into a Super Saiyan, to their fight when he was Majin Vegeta (half evil, half good), to their fight with Kid Buu. Bulma then explains how Goku became Super Saiyan 4 in the fight against Baby on the Tuffle planet: when Goku looked at the Earth from the Tuffle planet as if it were a full moon, he transformed into a Super Saiyan Great Ape because he absorbed the Blutz Waves that were naturally being created by the Earth, in the same way as if they are only created by the moon. 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 a regular Great Ape, theoretically he just needs to be bombarded with Blutz Waves to make the transformation to Super Saiyan Great Ape and then he can become a Super Saiyan 4. Bulma hypothesizes that Vegeta didn't transform into a Super Saiyan 4 while Baby was possessing him as a Super Saiyan Great Ape because Vegeta wasn't conscious. To create these Blutz Waves, Bulma has built a mobile version of the machine that she built for Baby on the Tuffle planet while she was possessed by one of Baby's parasitic offspring. (Source: Wikipedia)