Dragon Ball's Future Trunks is one of the series' most beloved characters and so it should come as no surprise he has been the center of several plot threads in events of the alternate timeline series, Super Dragon Ball Heroes. Because of his repeated prevalence, it seems as though Trunks was once slated to receive the strongest mortal Super Saiyan form, Super Saiyan 4. This, however, turned out not to be the case as eventually Trunks was given the power of Super Saiyan God. An interview has since revealed the exact reason Trunks never became a Super Saiyan 4.
Super Saiyan 4 has always been a highly exclusive form. Even in the Dragon Ball GT canon, only Goku, Vegeta, and their fusion counterpart Gogeta ever received the transformation. Since then the transformation has also been taken on by other characters like Gohan and Broly in the pages of Super Dragon Ball Heroes. Despite this, Trunks himself has still never been able to use this primal Saiyan power. Instead, he now commands the power of Super Saiyan God, one of Dragon Ball's more recent transformations. Trunks is uniquely one of the few recurring side characters to have achieved this form instead of being given Super Saiyan 4, and the reason for this sudden change in habit is shocking to say the least.
In Super Dragon Ball Heroes, Trunks is a member of the Time Patrol, an elite squad of time-traveling martial artists who are sent out on missions to correct the flow of time and prevent harmful alternate events from taking place. While on these missions, Trunks discovers a plot to bring the Demon King Mechikabura back to full power in order to destroy all time and space. In order to combat the growing threat of the Demon Realm, Trunks assembles a team of history's most powerful warriors and resolves to battle them. It is here where, while trying to give Trunks energy, a group of Z-Fighters consisting of Goku, Vegeta, Gohan, Goten, and Pan accidentally discover the Super Saiyan God ritual, granting Trunks the new transformation. An interview from a guidebook reveals that here, it was originally planned for Trunks to become a Super Saiyan 4 but an inability to come to a decision about his hair color led the idea being scrapped altogether and the instead going with Super Saiyan God.
Time Patrol Trunks Using Super Saiyan God Instead of Super Saiyan 4
Renowned translator in the Dragon Ball community @Cipher_db revealed that in the Dragon Ball Heroes 9th Anniversary book, it was revealed that Trunks was first meant to turn Super Saiyan 4 in this sequence of events. There were, however, debates about his hair color in the process of coming to a decision. Artists could not figure out what to do with Trunks' iconic purple hair in Super Saiayn 4. Up until then, every Super Saiyan 4 with perhaps the exception of Gogeta kept their base form's hair color upon transforming. The debate around Trunks specifically makes it seem as though there was no iteration of a purple-haired Super Saiyan 4 that could be agreed upon. Thus, the much easier decision to give Trunks Super Saiyan God, a form with a set hair color, was made and Super Saiyan 4 Trunks was scrapped completely.
While readers may never know what Super Saiyan 4 Trunks may have officially looked like had these plans gone through, Super Saiyan God is by no means a downgrade in the power or aesthetic departments. With Dragon Ball's more recent penchant for slapping a Super Saiyan form onto anything that moves, perhaps not all hope is lost for the hero from the future to achieve Super Saiyan 4 once a design can be settled upon.
Source: @Cipher_db