oh boy you have lots to learn on here. buy a matching sub for one of those. build a decent box and don't over power/clip the shit out of them and blow them again.
and as far as dougee goes if he wires the D4 up to 8 then he can wire the 2 8's back down to 4 i suppose but i don't see how he could get 8 ohms out of that. idk
EDIT: yea just re-read the op most of what you described is impossible i'd like to know how this stuff is wired
Think about physical woofers, actual coils. What happens when you wire an 8ohm coil in either series or parallel to a 4ohm coil, and then series/parallel to another 4ohm coil. I don't think it would work too well.
@OP, I guess that possibility could work, I didn't really think of it like that.
Is your box prefab ?
no i had a company here that sells subs make it for 250$
you paid $250 for a ported box for 2 12's? its only about $50 in parts. learn to build boxes and you'll save a crap ton of money