My penis is 4 inches erect. Will I be able to have sex or have kids?
I assume that you are fully grown, that is that you have reached stage 5 in you development.
Four inches long, erect, is smaller than the average, but not horribly short. There are men with penises which are less than two inches long when erect and they are still able to have sex and children.
A typical woman's vagina, when she is not sexually aroused is about three inches in length. A wife's body has the ability to adjust itself to the size of her husband's penis over the course of time. Strangely enough, a woman has less difficulty adjusting to a small penis than to a large one.
The areas where a woman gets her most pleasure from sex are near the opening of the vagina. Thus, the length of a man's penis doesn't matter as far as his ability to give his wife sexual pleasure. Deep penetration, while pleasurable to the husband, is actually uncomfortable for most wives. Thus you won't experience some of the initial awkward adjustments newly married couples have when it comes to sex.
Finally, even if you had a penis which could not reach to far end of your wife's vagina, a man squirts semen up to six or twelve inches from the tip when he ejaculates. Therefore distance doesn't matter. In addition, sperm swim. That is why some women have gotten pregnant from semen that was spilled near the entrance of the vagina.
There will be certain sexual positions which you will not be able to do with your wife simply because your penis won't be long enough, but there are more than enough to supply you and your wife years of pleasure. It is not a matter of how big your penis is, it is a matter of how well you use it.