Here's how the finale of Dragon Ball Daima explains why Goku never uses Super Saiyan 4 in the follow-up series, Super.

In the previous episode, a last-minute assist from the elderly Namekian Neva allowed Goku to canonically achieve the transformation dubbed “Super Saiyan 4” in Dragon Ball GT. Now, thanks to ...

and Trunks have all been able to access the Super Saiyan 2 form thanks to training with Gohan. Goku specifically unlocks an ...

As Super Saiyan 3 Goku struggled to deal any real damage to Gomah, Neva used his magic to spark a new kind of transformation within Goku. This turned out to be a new version of the Super Saiyan 4 ...