Here's the thing...you could have that conversation without the spanking, and the effect of the spanking itself is actually minimized *by* that conversation.
If you touch a hot stove, the action and the pain are directly connected. It's the immediacy of that connection that really makes the point. If you separate the cause and the effect with enough time to walk them to their bedroom and talk about it, then the connection is lost. The point is lost. *Especially* with a four year old.
If you aren't going to take advantage of the real educational value of pain, why use it at all?
(Yet another reason for my opposition is that spanking has to be sort of brutal to be meaningful, for reasons other than what I just said. More on that if somebody wants to hear it.)
If they understand what they've done wrong, then that's a wonderful opportunity to *enhance* the way they think, by then explaining why a relevant form of discipline was chosen. If they're old enough to understand that going out of bounds is bad, then they're old enough to understand that their punishment is that they have to stay in smaller bounds. If they can understand that they abused a privilege, they can understand why they're losing it. And if they're old enough to understand issues of trust, they're old enough to understand why it needs to be earned back, once lost.
It's thought that *this* is the reason why children who are spanked tend to lag behind their peers in cognitive reasoning. Because they miss out on those immensely valuable lessons and conversations in cause and effect (and empathy, responsibility and anything else that might be connected to a childish crime.) Mistakes and misjudgements are how we learn. If a significant number of opportunities to learn something are replaced with "Daddy will hit me," then whatever might have been learned from them is lost.