If you believe WinISD, there is nothing to be gained by changing the vent size from 4" to 5"
My enclosure net size is 10,702.1 cubic inches and if using the insulation boosts it by 20%, it is 12,842.5 cubic inches.
Tuning this enclosure to 28hz will give me pretty much flat response to 40hz and a slight bump up after that. If the insulation trick works it will be a slightly bigger bump but not much.
I have nothing to lose if it does not work and not much to gain if it does.
Why worry?
A 4-string 20-fret bass (like a standard Fender Precision) goes from E1 (41.203 Hz, second E below middle C) to Eb5 (622.25 Hz, second Eb above middle C).
https://www.quora.com/Whats-the-rang...guitar?share=1
The lowest note of a double bass is an E1 (on standard four-string basses) at approximately 41 Hz or a C1 (≈33 Hz), or sometimes B0 (≈31 Hz), when five strings are used.
https://en.wikipedia.org/wiki/Double_bass
This all hinges on WinISD being accurate.
I have uploaded my WinISD file, so if you want to play with the figures you certainly can.
It is here
http://www.datilcam.com/GregsBox.wpr
Let me know if you find some figures that are in error and I will run them on my version of WinISD.
Thanks
G