Goku being born with SSJ4 in fanfic would likely make him a force to be reckoned with from infancy. This could result in quicker power-ups, different battles, and maybe even a shift in the balance of power among the characters.
Yes, it is possible. In fanfiction, writers have the creative freedom to reimagine the origin of characters. Goku being born with Ultra Instinct can add a whole new layer to his character development. It could mean that his power growth would be different from the original story, perhaps faster or with different challenges along the way.
Perhaps his early birth gives him an edge in his ninja training. He could start learning advanced techniques sooner than in the original story. This might cause the village elders to take a different approach to him, either seeing him as a greater threat or a more valuable asset right from the start.
In the context of this fanfiction, his high - born power level would have a huge impact on his early days. He may grow up faster in terms of physical strength and combat skills. Since he's born with such power, he might not face the same challenges as other Saiyans in developing basic abilities. However, this could also lead to overconfidence. And perhaps he has to deal with the burden of high expectations from his community at a very young age. He might be constantly pushed to test his limits, which could be both a good and a bad thing for his development.
There would be a lot of implications for Goku's character growth. In the original story, Goku's journey to achieving Ultra Instinct was a long and arduous one filled with many battles and self - discoveries. If he was born with it, he would lack that growth experience. However, he could still have other forms of growth, like learning to control his emotions better while using Ultra Instinct. His relationships with his family and friends might also be different. For instance, Chi - Chi might be more worried about his safety since he has such a powerful ability from birth, and Gohan might look up to him even more. It could also change how Goku views himself. Instead of seeing himself as a constantly evolving warrior, he might feel a greater sense of responsibility due to his inborn power.
Perhaps in the fanfiction, Goku goes Super Saiyan early due to some special training or a unique emotional trigger. It could be that he witnesses the death of a different character, like Krillin, much earlier than in the original story. This extreme emotional distress causes him to break through his limits and transform. As a result, the Saiyan saga would take a different turn. Frieza might become aware of Goku's new power much sooner and plan different strategies to deal with him.
In the 'izuku born a year early fanfiction', it could mean that he enters the hero academy a year earlier. This would put him in a different class dynamic. He might face more challenges as he's younger and less physically developed compared to his classmates, but his determination could still shine through and he could form unique relationships with his peers and teachers.
Vegeta would also be an important character. His rivalry with Goku would intensify even more. Seeing Goku achieve the Super Saiyan form early would fuel his jealousy and his desire to become stronger. He might even try to find new ways to train or gain power to catch up with Goku.
Obviously, Goku is one of the main characters. Since it's about him being born with a high power level. And his parents might also be important characters as they would be related to his birth and the discovery of his high power level. Maybe there are also some Saiyan elders or trainers who play a role in his life as they would be interested in his power.
It could still be Orochimaru. His plans for immortality and power - seeking don't really depend on Naruto's birth time. So he would still be a major threat to the village and Naruto himself.