Hi i'm 18 years old and my penis is 4 inches when erect and i'm very worried about it?

Not much but it's OK: There's nothing you can do. But it really needn't be a big deal for you. Penis size is entirely unrelated to sexual function or fertility. It will never be a barrier to sexual health and happiness, for either you or your partners, unless you let it be. If some potential future partner cares about it, that will be good evidence for someone you don't want to spend your life with. Good luck!

Will that help ?: If I recall it correctly the great ape never exceeds 3 inches, but they seem to dominate their harem okay. You may or may not acquire some more size over time, or can certainly have surgery to change things if you choose. I would quit brroding over the issue.Talk with a professional counselor if this is making you miserable and consult with a urologist if you wish.

Erect penis: You are a bit under average but unless you have other health issues this is ok but if it's been awhile since last physical get one soon and discuss with physician