That sounds more like a home audio speaker as I generally do not see many built specifically like that for the car environment...I think you might even be better off by buying a home audio woofer such as an Onkyo-they're cheap and still retain good sound.
Actually, I setup 2 channel 4 way speaker in my car as: tweeter, Midrange, Midbass(woofer), subwoofer. thru audio control crossover 4XS. I search around the web but I don't see many out there and most of them is 6", 6.5" and 8". Wonder any arount 4".
infinity makes one. and your setup is the way to do it...i have mine setup that way too...i love audiocontrol...i have 2 audiocontrol dxs digital crossovers, the matrix line driver and the epicenter...its quality stuff.