Here's how you unlock him:
1. Complete the main story mode: This is the main requirement.
2. Gather enough Dragon Balls: After finishing the story, you'll need to gather the seven Dragon Balls.
3. Make a wish: With all the Dragon Balls, you can make a wish, and one of the options will be to unlock Super Saiyan 4 Goku.
That's it! You don't need to collect any special items or complete any side quests.