I think that most would agree that the "difficulty" of setups is related more to bird placement than simply the number of marks thrown. Factors such as wind, terrain, cover, and a dog's natural desire not to fight them all play into the "difficulty" of a setup. Factor in (no pun intended) retired guns (where the gunner is present when the mark is thrown and then disappears, forcing the dog to focus even more on where the mark is, due to a lack of reference from a white coat), and you add another element to your setup. And retiring the long marks is not always the most difficut. Many retrievers have problems "checking down" on short retired birds, causing them to drive through and get into trouble at other falls in the setup.
Triple with good bird placement beats a straight-up quad hands down Regards.