Would you really need 4? A single w7 would probably do the trick....

 

It's been that way in all of the vehicle-specific Audio forums I've spent any time in. One forum was the personal selling grounds for some guy in Colorado, every time a question was posted about a subwoofer setup he would post "buy the subwoofer setup I sell". Then other people started getting into the game, recommending his products as the best thing in car audio. He eventually became a mod, basically making his sub setup the official setup for that forum.