I don't agree with Bill Pentz, provided one keeps the connections to the 4" ports as short as possible.
Friction loss = ((Pipe friction factor) x (length of pipe) x (air velocity)≤) / (2 x (pipe diameter) x (gravitational acceleration))
So, it follow that if the 4" length of pipe is short, as well as the opening the effect of losses is not that great.
It also follows that for a given CFM a 6" line offers roughly 4 times the friction losses compared to a 4" line, meaning this is what is important.
Volume = Velocity x Area
so from 4" to 6" the area is:
4" = 12.6 square inches
6" = 28.3 sqaure inches'
To make things simple lets say the cross sectional area of the 4" pipe is 1/2 that of the 6" pipe. It means the velocity in the 4" pipe will be twice that in the 6" pipe.
In the friction formula above, friction losses are proportional to velocity squared, so it follows that the friction loss in the 4" pipe will be four times that in the 6" pipe and for any substantial length of pipe, that will kill the system
Pure mathematics is, in it's way, the poetry of logical ideas. - Albert Einstein.