After Goku turned into SSJ4 in Dragon Ball Daima, fans speculate that a condition featured in GT could explain why he doesn't do in Super.

Goku SSJ4 Daima Mod In Sparking Zero Spotted by TheGamer, modder EmiG84 already recreated the appearance of Super Saiyan 4 Goku in a beautiful way, making him look like the Daima's version.

SSJ4 transformation confirmed as canon in Dragon Ball Daima, sparking fan speculation. Goku reaches his limits and unlocks SSJ4 power without magic or tail in Episode 19.