"the average erect penis is about four inches in circumference "
She doesn't site a reference. I've seen that number closer to 5 in other studies.
It would have been nice if she mentioned that a man can naturally increase the girth .5 - 1.0 inches by a simple technique. Get a firm erection, and keep that erection for 20-30 minutes, 3 or so times a week. While erect, practice inhaling through the nose, and while exhaling slowly out your mouth, clench the kegel muscles and breathe out your mouth and expand your lower abdominal muscles out, almost as if you are breasthing out your penis. Works best sitting slightly reclined in a chair. One additional benefit of this technique is the breathing technique, if done when you are having intercourse, can allow you to last longer.
In the article too, take note of her description of the interior parts of the clitoral structure. This is one reason many ladies like having the labia majora area massaged (bulb of vestibule in the pic below) during intercourse (easiest in missionary, also in doggystyle if the guy can handle mutliple tasks at once without losing control).