Bill, "it's very commonplace in professionally-designed duct systems to reduce to 4" ports" but in most home systems it is absolutely not necessary and actually reduces the performance of those systems.
The reason Oneida and others do it with dust collection systems and also why HVAC system designers reduce the size of the duct (check it out in the nearest open ceiling restaurant with rounc ductting), is because they have multiple openings (ports, registers, etc.). Reducing the diameter of the drops has two purposes- it balances the system between drops and helps maintain sufficient velocity so the dust and chips stay in suspension if more than one drop and its blast gate are open.
If you have a typical home shop and are a typical woodworking you use only one machine at a time. If you reduce the diameter of your drops (and ports) you must realize you are sacrificing performance, big time. With the same blower, the CFM possible through a 4" duct is considerably less than that through 6" duct as many people who originally ran 4" ducting have found. For collection from one machine at a time it is best to run as large a duct, as far as possible, without reducing at a drop or port. The object is to collect as much dust as possible at the source. Again, this is for a home shop not a commercial one where ductwork runs long distances and supports multiple machines running at the same time.