how long does it need to be?

4 inches is fine. Though if he is stil under 20, there is a good chance it may grow more. But even if it doesn;t, all he has to do is practice and he can be as good at sex as any guy and maybe even better than a lot whoe have longer ones.

Hell be able to reproduce, however he needs to find a woman who will be happy with a 4'' dick. I hope it will still grow. Sorry Guys, but size does matter.

there r two thing one is sex performance and other is ur reproductivity

performance depends on ur stamina , ur mood , and how long u can hold and not merely on length of ur dick

second thing reproductivity is absolutely independent of lenght

so tell him to be stress free

be happy and go lucky

it is not the length but the strength that matters.............

Tell him not to worry. The sperms shoot at the average speed of one meter per second so they will reach the target.

He'll be able to have kids. Either the natural way or via insemination. Tell him to worry about other things.

He can have kids with no problem, but finding a woman he can please...maybe more difficult. And in my opinion it's not just length, but girth too.

Tell your friend that he should not be worried about his size. 4" is OK. It is just a little under the average.

I am a girl, but I don't see an issue. As long as he has the little swimmers in there, it should not be a problem!

Don't be foolish. The length has nothing to do with the ability to reproduce.

Even if he have a small penis, the sperm is shooted with so much pressure and force that it doesn't matter how big or small it is.