I disagree with you. To move the same amount of air through a 4" pipe as through a 6" pipe, the velocity would have to be 3^2*PI/2^2*PI = 9/4 as fast. Thus air with a 6" speed of 4000 ft/min would have to travel 9000 ft/min.
Drag in air varies with the cube of the velocity, so the drag is (9/4)^3 = 11.4, meaning that one foot of 4" pipe would take 11X as much pressure drop to move the same amount of air as in a 6" pipe. Said another way, using 6' of 4" hose would equate to 66' of 6" pipe.
With dust collectors we only have about 9"-12" of vacuum to deal with, so the result is less air. If I was using a shop-vac that could draw 80 or 90" of vacuum the effect would not be so pronounced.
Sorry about all of the technojargon, Mike. I had a class in HVAC design back in engineering school... in the dark ages.. B.C.... Before Computer.
I've just been researching on from what I"ve learned from Bill Penz of fine dust and shop safety. Its a really good site from someone who explains his medical problems due to dust in a technical manner and isn't trying to sell you anything. He also offers free plans for a built-it-yourself cyclone dust collector based upon his findings. I used to not be bothered about picking wood-b**gers the day after woodworking but I'm trying to change as I've become AARP-qualified!!
I may need to make my own or use 2 ea 4" runs if I decide I need the extra air.
One thing though.. it can't collect more than 100% of the dust; if 4" does it, it does it!! I just want to ensure enough volume to get the fines too.