Yeah, not really stumping me too much, either. I have a rough idea of what a "4" is. Less than 4 is smaller. Greater than 4 is bigger.
Based on what the site said, the incremental differences between 2, 3 and 4 are tiny. But from 4 to 5 to 6, you're taking leaps. And yes, it was based on how much to charge for the difference in chunk of briar, assuming, I guess, guys making tiny pipes weren't lopping off a lot of root from a giant starting block. Or other side of the sales counter, rating it on how much briar the customer is buying in a finished product, regardless of scrap left on the maker's floor.
Either way, hole size wouldn't make much difference. You can whack as much out of your briar for a hole you want. And I guess that's *why* you can't get bowl dimensions. It's not about bowl dimensions. (Within allowances. Like, you can't put in an 1-1/2" hole in a 1-1/4 inch pipe.)
So, referencing a Dunhill number to compare how tobacco smokes in a pipe... that doesn't make sense to begin with, does it? That's not what the number was for. But a lot of people seem to want to use it that way. For instance, I've read it used in a tobacco review saying that's what you could expect from a "number 4 pipe." I assumed from the context it meant, "In a typical pipe." But maybe it would have been more accurate to just say "in a standard pipe" rather than make people search the Internet, call a friend, and ask the audience.
I guessed from the reference that the author was speaking about a 5 1/2-incher with a Grabow-sized bowl -- myself never having handled a #4 Dunhill. I'm not running out to "get experience" by buying a bunch of Dunhills or bugging people to show me theirs.
So was I close? Close enough, I think.
Horseshoes, hand grenades and Dunhill sizing, my friends.