The trials and tribulations of Vegeta throughout his Dragon Ball career have led him down a path of ever-increasing power, which usually manifests itself in the form of an all-new transformation–and none are more important than Super Saiyan 4 (but not for reasons one might expect).
The first major transformation achieved by Vegeta was his Great Ape form. While this was one fans have seen from both Goku and Gohan in the past, Vegeta’s was different. Instead of becoming a mindless, destructive beast, Vegeta retained control over his mind as a Great Ape, effectively proving to be the master of this Saiyan form even if he wasn’t the first Saiyan shown utilizing its power–which is a trend that would continue through DBZ and into Dragon Ball Super. In Super, Vegeta unlocked Super Saiyan God without the use of the Super Saiyan God ritual (like Goku), making his ascension of power that much more impressive. While Vegeta consistently takes pre-existing Saiyan transformations and upgrades them, that fact actually doesn’t play a role in why Super Saiyan 4 is Vegeta’s most important form, as the real reason is way more meta.
Super Saiyan 4 Changed Vegeta’s Entire Mindset with 1 Thing: Fusions
In Dragon Ball GT episode 59 (written by Toshinobu Ooi, directed by Osamu Kasai, produced by Toei Animation), Vegeta s Goku as a Super Saiyan 4 after Bulma blasts him with her Blutz Waves Generator. Blutz Waves are types of photon particles that are transferred by the light of the full moon (or really any other celestial orb, as it turns out), and Bulma figured out how to make that energy even more concentrated. Usually, a Saiyan needs their tail to become a Great Ape after absorbing the Blutz Waves from the full moon, but with Bulma’s Blutz Waves Generator, no tail was necessary. Since becoming a Great Ape is required to unlock Super Saiyan 4, this machine proved integral in Vegeta reaching the transformation. Then, once he unlocked SSJ4, Vegeta does the one thing that he would seemingly never do: Vegeta asks Goku to fuse with him.
The whole reason Vegeta wanted to become a Super Saiyan 4 in the first place was to help Goku defeat Omega Shenron, and Vegeta knew once he made the transformation that he’d need to fuse with Goku to achieve victory. Under normal circumstances, Vegeta would not only hate the idea of fusing with Goku, but he’d want to take a shot at battling the villain himself first before resorting to fusions. In fact, in the past, it’s always been Goku who has to talk Vegeta into fusing with him–whether it be during their fight with Majin Buu in DBZ, or their battles with Zamasu or Broly in Super, and even in the movie Dragon Ball Z: Fusion Reborn. Vegeta’s Saiyan Pride demands he claim victory over his opponent himself, without anyone’s help. However, when he became a Super Saiyan 4, that mindset flew right out the proverbial window.
As many fans of Dragon Ball know, Goku and Vegeta fusing to become either Vegito or Gogeta is the obvious solution to battling god-tier opponents, and every minute they spend not fusing can sometimes be frustrating to watch. In this GT episode, Vegeta was aware of that, which is why he was the one to suggest it in the first place. This metamorphosis of reason and logic over stubbornness and pride is a huge step for Vegeta, especially in the meta sense, as his character has been known to go through significant regressions in the past. This productive character change happened alongside his transformation into Super Saiyan 4, making it Vegeta’s most important form in Dragon Ball.