In Vegeta's dragon universe mode {Majin saga} enter every possible areas and battles before each dot after that instead of fighting kid Buu you will fight Brolly defet him and after that Vegeta will sense a new energy which will be SS4 Goku defet him then you will obtain the skill SS4 Gogeta fusion. you can also buy the skill for 60,000 zeni in the skill shop after obtaining the Black Membership card.

In Vegeta's 2nd DU, as soon as Vegeta is resurrected after Fat Buu, go to muscle tower on the ice island. There you will automatically talk to kibitoshin about Broly. Then go fuse and fight Buu at the dot. As you go to fight kid Buu Goku will instead send you to find and kill Broly. Once you can move, instead of flying to Broly, fly around the chain of islands to the SE. Here you will see a event with Vegeta complaining about Goku while King Kai makes it so Goku can here Vegeta complain. After that go kill Broly. After Broly, Vegeta will sense a new power but it's not evil. instead of heading to the new dot to the south, go talk to piccolo at kami's tower, he will question him. After, go towards Baba’s palace. in a "plains" area you will talk to Bulma about Goku turning ss4. Then she tells you to go home (West City) for a surprise. She gives you the SS4 capsule. Now go beat Goku at the red dot to the south. If you beat SS4 Goku, you will get Vegeta’s capsule for SS4
gogeta