Salman,
Your friend at 4 inches is on the small side IF that is when he is erect. If that is when he is soft he has nothing to worry about.
He is nineteen - so he could still keep growing for another 3 years.
There is no food you can eat, no exercise you can do, no vitamins you can take, no medications ------you are born with certain genes and those genes determine how big your penis will be.
It is very likely that you are the same size of your father. And your father the same size as his father.
Learn to use your penis well and no woman ( or man ) will care what size it is.
Susan