I don't see the distinction between punishment and discipline when it comes to kids.
Ideally, we punish our kids to teach them internal discipline, and to teach them that there are negative consequences when they break family and social rules. In other words, we use punishment to instill in them the discipline necessary to learn self control, and to deal properly with parental, social and legal authority.
I just don't see how spanking contributes to that process at all.
Some children may never need much, if any spanking. If you are speaking of your experience with a single four year old, I'd suggest that your experience is very narrow.
What you can do with one may not work as well with four. But you may not be able to see that. Yet.