I really like 36hz tuning on them. I installed 2 9515g's in my friends blazer, 8.25 2 6" aeros tuned at 36, and it plays fully from 25hz-65hz with authority. I would use a bit larger then 4, just IMO though
Like 4.5ft or so....depends what your listening to! Honeslty, best blend IMO for you is going to be 4.25@ 37hz, I havent used that exact box but being right in the middle, it should work great.
4.5-5.5 net should be good, and 40hz tuning will deffinalty play under 40 with some authority still..... be happy the newer 9515s take smaller boxs the old ones i used were better in 6 net and up.... but in my case they ive always went back to 39-40 hz tuning....no matter the box..
look around for typerbass's threads from a your or 2 agohe had one of those he ripped apart with the bass, 4 9515s and i forget the amps..... it was on realm b4 too, but he also had aeros and no back seats....