Many dust collection systems suffer due to poor ductwork designs. There is a large amount of misinformation about what the proper size pipe is to run. In reality, proper pipe sizing can get complicated and depends on many variables; tool size, air requirements, length of pipe run required, number of machines running at one time etc.
A very common and very disastrous problem in duct design is to simply run all 4" diameter pipe. People run 4" pipe for several reasons... usually not the right ones! Most "import" tools and chip collectors are outfitted with 4" diameter ports, 4" pipe is common, 4" pipe is inexpensive, 4" pipe is the most common size of PVC pipe. None of these reasons means that 4" pipe is the "right" size to run for optimal performance.
Tools are sold with 4" ports because the chip collectors have 4" ports. That doesn't mean that a tool wouldn't be much better off with a 5" or 6" port.
Lets look at an example of a small shop that has a large tool in it. In this case a 20" planer. Now I realize that not everyone has a 20" planer but more and more small shops are able to afford "large" tools. Planers, Drum sanders, more powerful table saws all require a lot more airflow than they did a few years ago.
This simplified layout only shows the duct run from the system to the planer. We will assume this is the run in question or the worst case for this shop.