On WinISD, does a 4 inch port mean 4 inch internal or external diameter??
I think PVC is measured for internal diameter. This allows for calculations of flow rates.
The port doesn't have to be 4". It could be 3.75" or 3.85"
It would just require a different port length for the same tuning freq.
Go to Lowe's and check out the PVC sizes to be sure.
Pioneer 8600 - Profile Amp-Infinity Speakers - L7 Subwoofer