If someone actually said that, I would assume they are crazy and ignore them. However, in the scenario that I know they are telling the truth, I would go back to the age of 4 and change a lot of things. Even though a 4 year old doesn’t have a lot of agency over their lives, they can still influence things for the better.
I would tell my parents that I travelled from the future, and prove it by telling them of some future events that aren’t that bad, waiting for it to happen. I would tell my parents what stocks to buy to get rich.
I would be sure to become a good sibling, because as a kid I was completely self absorbed in my own misery and ignored everyone else.
Also, I would spend more time with my grandmother. I never appreciated visiting her, because we lived in different cities and I always hated car travel, but she was such a wonderful human being, so I really wish to have had the chance to spend more time with her before her passing.