Dragon Ball Super isn't always about Goku, and the Black Frieza Saga has the perfect chance to highlight another, more ...

In the episode's final minutes, Goku explained to Vegeta that he achieved Super Saiyan 4 through rigorous training after their battle against Majin Buu and all his troublesome variations.

In a wild twist, Goku confirms in the series finale that he had always had the power to transform into this state, thanks to training in the background following the fight against Majin Buu.