Hi sploo,
What machinery you working with; may be you will only need to use the 4” pipe? If you are using a planer thicknesser you may have to have 6” pipe due to the long shavings they tend to make, to avoid clogging (if you were to use twin pipes shavings would clog at the joint.
Also if a machine has a 4” out let then you should be ok with the 4”.
Question, why are you planning to convert the system to cyclone, what would be wrong with the system as is?
As for sound proofing, would the extractor be louder than your machinery? The sound from an extractor is quite low pitched so you may not get complaints from others.
John