Dividing fans since its debut in 1996, Dragon Ball GT has largely been dismissed as no longer canon to the overall Dragon Ball due to Dragon Ball Super continuing the manga/anime property. However, the short-lived anime continuation does still have its own vocal fanbase, with characters and concepts that have since lived on beyond its run. The most notable of these concepts is Super Saiyan 4, the final Super Saiyan transformation seen in the series -- and one of the most controversial.
GT introduced the genetically engineered villain Baby, who was able to possess the minds of those he took control over. Deciding to use Vegeta as a host body, Baby easily defeated Goku, even at Super Saiyan 3 and after the Saiyan hero had regrown his tail to unlock his latent power. From their battlefield and the new Planet Plant, a beaten Goku stared into the Earth's horizon, triggering a transformation into a Great Ape as the sight of the full moon. However, having long since unlocked the powers of a Super Saiyan, Goku's Great Ape form took on a golden fur while becoming even more destructive than ever before.
Goku's granddaughter Pan helped calm her grandfather down, allowing the older Saiyan to fully master the Golden Ape transformation and consolidate it into a human form he dubbed Super Saiyan 4. No longer sporting a Super Saiyan's iconic golden hair, Goku was now a combination of his normal form with a more bestial state, reflecting the transformation's simian origins. Goku's body was covered in bright red fur, while his hair was long and black. By the end of the series, Vegeta similarly achieved this transformation after Bulma immersed him in Brutz waves, triggering his own Golden Ape transformation which he quickly mastered. Upon his debut as a Super Saiyan 4, Goku and Vegeta used the Fusion Dance to merge into the composite warrior Gogeta. Gogeta was also Super Saiyan 4 due to his components.
Due to the drastic visual differences and Vegeta skipping Super Saiyan 3 altogether, reception to Super Saiyan 4 was mixed, with some praising its unique design and inspiration from Saiyan genetic origins while others were put off by its departure in appearance and methodology through which it was achieved. Dragon Ball Super ignored this form entirely to introduce its own Super Saiyan God, Super Saiyan Blue and Ultra Instinct transformations -- through the canonical Legendary Super Saiyan form in the anime film Dragon Ball Super: Broly was conceptually similar to Super Saiyan 4.
While Broly lacked the combat experience of Goku and Vegeta, the Saiyan warrior was more than capable of fighting against his more seasoned counterparts in his base state, even while each of his opponents were transformed into Super Saiyan Gods. Broly's father Paragus theorized this was due to his son channeling the raw primal fury of a Saiyan Great Ape, as evidenced by Broly still possessing his tail. Similar to a Great Ape, Broly lost complete control of his actions to a berserker fury, which augmented significantly when he transformed into his Legendary Super Saiyan state.
Since the conclusion of GT, Super Saiyan 4 has only been seen in non-canonical video games, merchandising and the non-canonical tie-in anime series. While Broly was able to channel the power similar to a Super Saiyan 4 in his canonical debut, the actual controversial transformation is unlikely to be re-integrated into the franchise's official canon any time soon.