Dragon Ball Super introduced fans to Super Saiyan God, a form that existed separate from the well-known Super Saiyan transformation. When Goku turned Super Saiyan while a Super Saiyan God, he discovered the Super Saiyan God Super Saiyan form -- often called Super Saiyan Blue. Super Saiyan Blue proves that Goku is capable of separate transformations that can stack, a concept taken to its extreme when Goku stacked Super Saiyan Blue with the Kaio-Ken technique. This pushed Dragon Ball Super down the path of non-linear transformations.
Can the Super Saiyan God form stack with Dragon Ball GT's Super Saiyan 4 transformation? Akira Toriyama established that Super Saiyan 1, 2 and 3 are all one linear line of transformation. However, Super Saiyan 4 is an inherently separate form from the prior Super Saiyan transformations due to how unique the requirements are to become a Super Saiyan 4. Given Super Saiyan 4 is a unique form, is it possible for Goku to become a Super Saiyan God Super Saiyan 4?
What Makes Super Saiyan 4 Separate From Other Transformations
Despite what one might assume from its numerical name, Super Saiyan 4 is not a powered-up version of the standard Super Saiyan form like 2 and 3 are. The only way to transform into a Super Saiyan 4 is to first transform into a Golden Oozaru, then gain control of it. This makes Super Saiyan 4 more akin to the Super Saiyan God form than the prior three Super Saiyan levels. Both 4 and God are easier to maintain than Goku's Super Saiyan 3 form. All of this indicates that Super Saiyan 4 is separate from the Super Saiyan transformation line.
Can Super Saiyan God and Super Saiyan 4 be Stacked?
By this logic, there is no reason why Goku would be unable to stack Super Saiyan God and Super Saiyan 4. However, Goku and Vegeta never attempted to stack Super Saiyan 4 with prior Super Saiyan forms back in Dragon Ball GT. Goku and Vegeta never even attempt to stack the Super Saiyan 4 with the Kaio-Ken technique. However, when we see Super Saiyan 4 Gogeta, Gogeta's hair goes from black to red, much like a Super Saiyan God's hair.
It is possible that stacking Super Saiyan God with Super Saiyan 4 might result in a Super Saiyan 4 form similar looking to Super Saiyan 4 Gogeta. The only characters who could potentially stack these forms would be Xeno Goku, Xeno Vegeta and Xeno Trunks from Super Dragon Ball Heroes. Not only do these characters already have the ability to turn Super Saiyan 4, but having met Goku and Vegeta from Dragon Ball Super, they also have access to information that could lead them to unlock the Super Saiyan God transformation.
How Strong Would Super Saiyan God Super Saiyan 4 Be?
How absurdly strong could a Super Saiyan God Super Saiyan 4 be? It seems likely that the two forms combined would become comparable to Hyper Instinct Goku or Super Saiyan 4 Gogeta in terms of raw power. However, this form might be difficult to maintain, since Super Saiyan 4 is harder for Goku to maintain than Super Saiyan Blue. Even so, if he trains to pull this off, Super Saiyan God Super Saiyan 4 could be the strongest form ever for Goku.
