Try 7 cubes net to start with, utilizing a couple or several flared round ports, and then you would be a great place to start your testing and more testing. I bet when/if you get down into the 6 cubes net range, and whatever tuning you prefer, you'll be at the loudest they're going to get with a decent sounding response and passband.
But, IMO, I'd sell those four drivers and purchase a pair of more capable 12" or pair of 15" drivers that can use the 1.6kW available, much better. And well, they'll get louder I'd bet, and sound great while doing so. Oh, and if you choose to, the pair of 15" drivers will use an enclosure essentially the same size as the four 12" would normally use, but not always.