Super Saiyan 4 doesn’t turn Goku back into an adult. Dragon Ball Daima episode #18’s interpretation of Super Saiyan 4 is probably the most organic way that it could have been brought into canon.

Dragon Ball Daima hit Goku ... But when Gomah was holding Goku in place with his magic, Goku realized that he can’t really hold back anymore and fully unleashes his Super Saiyan 3 form.

Dragon Ball Daima ... debut of Super Saiyan 4 Goku is a big deal, it still was nowhere near enough to deal any kind of real damage to Supreme Demon King Gomah as he brought Goku back to his ...