Witnessing new transformations and seeing characters reach new power levels is one of Dragon Ball's biggest appeals. There is constant speculation in the community over which character will reach a new form and what powerful new abilities they'll gain. There are also heated arguments about which transformation is the most powerful or the fastest. Sometimes these debates are over which transformation is the best designed, and in this regard, one transformation is sadly overlooked.
Although it may not be the strongest and comes from a controversial series, Super Saiyan 4 stands out from all the other transformations in the franchise. Dragon Ball GT was a polarizing series in the fanbase with very vocal supporters and detractors. Regardless of one's opinion about the story of Dragon Ball GT, it can't be denied that Super Saiyan 4 left an impression. Even now that Dragon Ball GT has been over for years, the transformation still makes appearances in other series, like Super Dragon Ball Heroes. Fans are still drawn to the look of Super Saiyan 4, and for good reason.
A Unique Look for Saiyans
When compared to all the other Saiyan transformations that have been showcased in Dragon Ball, Super Saiyan 4 is clearly the odd duck out. Super Saiyan 4 breaks away from the established trend of hair color change and special aura. This departure bothered some fans, but it's exactly why the transformation is so great.
Super Saiyan 4 is far more memorable than a form like Super Saiyan 2, which is only slightly different than the base Super Saiyan. Super Saiyan 4 carries more personality and reveals aspects about the characters. From a visual standpoint, Super Saiyan Blue looks more like a rehash of previous designs. When characters power up and reach a new form, the design should signify that change in power. The bigger the change in look, the better the change in power is communicated.
Of course, the more different the look, the better the design is not a hard rule. There are plenty of examples of extremely unique transformations that don't make for great designs. Frieza transformed a number of times while on Planet Namek, but it was only his final form that became memorable. All of his other transformations were largely forgotten and ignored by the fandom. However, Super Saiyan 4 manages to be something new while still feeling like a Saiyan transformation.
A Natural Evolution of Saiyans
If one were to show a person who knew nothing about Dragon Ball an image of a normal Saiyan and an image of Super Saiyan 3, they might not be able to tell that these two forms are related in any way. Super Saiyan 4 fixes that issue in an elegant way. This transformation feels like the natural next step in a Saiyan's evolution.
Super Saiyan 4 maintains the black hair and monkey-like tail that all Saiyans are born with. A layer of fur has grown across the body, making Super Saiyan 4 look even more monkey-like. The hair is wilder without becoming ridiculously long and crazy, as with Super 3. The whole design of Super Saiyan 4 feels like the meaner, bigger brother of a normal Saiyan. It's something different and more powerful, but still in the same family.
A Call Back From the Past
In Dragon Ball GT, Goku reached the level of Super Saiyan after gaining control of his Golden Great Ape form. The Great Ape form had mostly stopped being important in the main series after Vegeta transformed into one during the Saiyan Saga. After this, the Great Ape form is viewed as more of a hindrance than an actually useful transformation. Dragon Ball GT makes the Great Ape form useful again by merging it with the Super Saiyan form. Super Saiyan 4's design draws from Dragon Ball's roots to create something great.
Dragon Ball began as a semi-adaptation of Journey to the West -- a famous Chinese novel focusing on Sun Wukong, the Monkey King. Goku was originally going to be a full monkey to better fit the story of Sun Wukong. That design was eventually changed, but Goku managed to keep his tail. By giving Goku more monkey-like features in Super Saiyan 4, he is returning to his original design. In other words, Super Saiyan 4 makes Son Goku the Monkey King he was always supposed to be.