Actually, I let them slide everyday.
This one was a little weird to me, because dubbels and trippels have a style associated with them, but I have absolutely no idea how a quadrupel tastes like, and I've been carrying various Belgian beers for a while. Anyway, I believe the story is that back in the days before they could measure ABV, they would "double" or "triple" the ingredients to make them stronger. The nomenclature likely stopped at triple, due to the importance of the Holy Trinity, as after all these were religious monks brewing the stuff.
As we're now on the "etymology of beers", the Hennepin beer was named after the Belgian priest who discovered Niagara Falls. I'm sure they chose it to combine the ideas of both New York and Belgium.