All circumcisions are not created equal. There is no dotted line on a baby's penis saying "Cut Here" to tell doctors exactly how much foreskin to remove, so every one comes out different. There's also no way to predict, at birth, how large a boy's penis will be when he's an adult, so again, it's impossible for them all to turn out the same.
Even among intact males, some have more foreskin than others. None of it is "extra", and more than a person with large earlobes or long fingers has "extra".
As Bandgeek said, today doctors are more often erring on the side of removing less instead of more, because of the many problems caused by cutting too much. I'll bet the head appears when he has an erection - which is exactly what is supposed to happen.
Your DP is dead wrong. Your son's penis is just fine - he does not have a medical condition, therefore he does NOT need surgery. Perhaps this will help:
http://www.medicinenet.com/surgery_questions/article.htm
It is a list of questions to ask before having surgery. Most people don't think of circumcision as an operation, but "just a snip" - but a 4-yr-old will require general anesthesia (which has it's own risks), and a significant healing period, though this time he can tell you when he needs more pain medication.
My two sons have had a total of 7 operations, and we took every one very seriously. In two cases, we waited years before having surgery, because we didn't want to do it if it wasn't absolutely necessary. By the time our kids were 3, we took their opinion into consideration (my 3-yr-old had a hernia, which was painful, so he wanted Dr. T to fix it and make him better). My other son made the decision to have a ruptured eardrum repaired at age 5, since he was the one who had to deal with an earplug in the swimming pool. If he had not wanted it done then, we would have waited.
I doubt your son, at age 4, would be too thrilled about having part of his penis cut off, when it's not giving him any trouble.