Super Saiyan 4 may not officially be part of the Dragon Ball canon, that is unless one Dragon Ball Super: Broly theory is confirmed someday, but it remains one of the most memorable and favorably looked upon additions from Dragon Ball GT. Thanks to the non-canon adventures of Super Dragon Ball Heroes, Super Saiyan 4 has made its way back to the franchise with the alternate Xeno versions of Goku and Vegeta. But it turns out that there initially were plans for Trunks to get a Super Saiyan 4 form as well.
In a recent interview shared in the special guidebook celebrating the 9th Anniversary of the Super Dragon Ball Heroes arcade game, it's revealed that Trunks almost got a Super Saiyan 4 form but the team couldn't decide what kind of hair to give him.
やはりトランクスも超サイヤ人4にする案はあったんだな！— ウルティマ (@Lord_Towa9377) November 7, 2019
As the interview blurb explains, Trunks was supposed to get this form alongside his Goku and Vegeta Xeno counterparts given how integral Future Trunks has been to the Xeno storyline in the franchise, "We decided 'For sure, at the end it would be Trunks.'...At first there was a plan to go to SSJ4, but there were various problems such as whether the hair color was purple or black, and it became a lot."
So because they couldn't decide whether or not Trunks' hair color would change to look more like SSJ4 Goku, Vegeta, and even Vegito, they decided to go a new route an introduced the equally as new Super Saiyan God form for Trunks. It's pretty funny that it all came down to this design choice, but with a card game with many collectible characters aesthetic really is everything.
Although with Trunks getting so much shine in the side projects, it certainly opens the door for other characters such as Gohan or Piccolo to potentially get their own power upgrades someday. If Trunks nearly got a Super Saiyan 4 upgrade, why can't everyone else? If it's just hair, then Gohan is definitely worthy!
If you wanted to check out the promotional anime series for yourself, Super Dragon Ball Heroes Episode 17 is now live. It's titled, "Ultimate God Killer! The Birth of Hearts!" and the synopsis reads as such, "Hearts has finally evolved into his ultimate form. As Jiren and Hit come to the rescue, Goku and co. start an all-out battle! The final battle that decides the fate of the universe begins!"