If you look at this graph, you'll see that the larger the change in cross sectional area is, the more insertion loss you will achieve. Therefore, if you are feeding your silencer box with a 4" RD duct, your cross sectional area is 12.57 sq in.I can get this well under 300fps with 4" round ducting. My plan was to make the internal cross section of my silencers 4x4 square. Do you think that would be sufficient?
If you plan to have a 4"x4" square path inside of your silencer, your cross sectional area is 16 sq in. That is basically no change in cross sectional area. Essentially this would not offer any real insertion loss but instead you would have tight 180 degree turns which would just increase your static pressure. Granted, the turns would help with insertion loss, your low frequency insertion loss is achieved with the impedance mismatch.
Having said that, you should have at LEAST a cross sectional area inside of your silencer of 25 sq in. equaling a 5"x5" path. Again, you'll achieve way better insertion loss and less static pressure with a larger path. Remember, this 5"x5" or larger path is the size inside of your duct liner! Also, I recommend making the path even larger around the turns (around the ends of your baffles) to help with static pressure.
Greg