It's been a quarter of a century since Dragon Ball GT came to an end and Super Saiyan 4 is still one of the most popular transformations in the Dragon Ball universe. Despite the fact that it is also the most controversial Dragon Ball series and that SS4 is not even canon anymore, the power-up was so different and had such an amazing design that fans keep loving it.
Super Saiyan 4 still shows up in the games, fan-fiction, and non-canon stories. That's not a coincidence. But with how much the series has moved passed the form, can fans ever expect the form to come back in some way?
An Outdated Power Level
One of the main reasons why Dragon Ball GT was labeled as non-canon after Dragon Ball Super was the dramatic difference in power. Dragon Ball GT was set 5 years after the end of Dragon Ball Z, 10 years after the defeat of Majin Buu, and was an original anime story, unlike any other Dragon Ball Series. At the time it served as a sequel to the events of Z, but it missed Akira Toriyama's writing, although he had designed some of the new characters. As a result, GT got mixed reviews among fans and critics, most of them rating it as the worst Dragon Ball anime.
However, it had great ideas one of them being the creation of Super Saiyan 4. Fans in the 90s were wondering how long will Goku's hair grow for the next transformation, which seemed kind of ridiculous after Super Saiyan 3. The solution was going in a completely different direction with a redesign that brought back their true Saiyan form with ape-like features. It was an instant success and the most powerful transformation by far, until Dragon Ball Super happened. Akira Toriyama decided to continue the story precisely in the 10-year gap before GT and in no time it already had Gods of Destruction and Super Saiyan God, both way more powerful than Super Saiyan 4, making it rather unnecessary for Goku and Vegeta, who are playing in a better league.
As usual, the villains and challenges must be in the Super Saiyan God, Blue or Ultra Instinct level from now on, so it wouldn't make sense for them to start developing and inferior transformation. The only thing we could expect is a design that mixes Super Saiyan 4 and Ultra Instinct as a new form, but that would be completely different and unrelated to the GT transformation in anything other than the looks of it.
The Hope of Universe 6
Unfortunately, seeing Super Saiyan 4 in any of the show's common heroes looks impossible. Goku, Vegeta and Future Trunks are above it power-wise, Gohan's power-ups go in a different direction, and Goten and present Trunks are too inexperienced to achieve this transformation. Specially considering how difficult it was to become Super Saiyan 4, with the Golden Great Ape previous step or the exposure to Blutz Waves, so what's next. Most theories go through two parts of Super's canon: Broly and the Universe 6 Saiyans.
Broly, like Super Saiyan 4, was always a fan-favorite, but until very recently he remained as a non-canon character only appearing in movies, spectacular ones, but that weren't a part of the story in Dragon Ball Z. Dragon Ball Super: Broly changed that forever. The Legendary Super Saiyan know for losing his temper and being obsessed with Kakarot was re-worked into the canon plot with this movie. Something similar can be done with Super Saiyan 4, a form that could eventually be added to Broly himself, after showcasing Broly's subconscious use the Great Ape form that could lead to unleashing this ultimate power. This will make perfect sense becuase Broly's off the charts natural ability could put Super Saiyan 4 at the same power level of Goku's Ultra Instinct, but it will be a different design as it would have to be fused with the Legendary Super Saiyan style.
Nonetheless, the most realistic and probable option is the Universe 6 Saiyans. Not only they have their own Broly in Kale, but they are a whole group of Saiyans that has been following the traditional Super Saiyan power growth, and they are also more powerful in those forms than the Universe 7 equivalents. The only thing standing in the way to bringing Super Saiyan 4 to Cabba and company is that they are born without tails. However, Vegeta managed to find a solution for that in GT, and apart from that, it can simply be a different method to gain this power in Dragon Ball Super.
Instead of continuing to copy Goku and Vegeta, it would be an interesting way for Cabba, Kale and Caulifla to grow on their own, with and unique path, while also giving the fans the return of a beloved transformation that has been lost in the canon story. They could also use it to go deeper in why they don't have tails and present more characters that will add depth to the twin universe which might have a big role in the future of Dragon Ball Super. The opportunity to canonize Super Saiyan 4 is there, and it's definitely likely that Toriyama has been thinking about it. The only thing left for the fans is to wait and see if it ever comes back and how.