Here is the down low---it's gonna suck. Pure and simple. Interchangable ports? Why? My frind had his car metered and it hit 144.2 with only 1 15 inch JL W3 and it was not even three hours broken in, and it was sealed.
So here you are now, your back seat I am thinking has been crudely ripped from it's home in your car aka the "Granny Moblie?" Well I don't know about you but my 3.5 ES is more than what my grandma needs.
Anywho. I think that you need to take some time and find out what it is that you are really after- SQ or SPL. If it is one or the other or both it will guide you in the direction that you need to go, which btw is not with a pair of 18's. 18's are pointless. They have such large amounts of cone area that it becomes uncontrolled and distorted, plus I can almost gurantee that you barely have the necessary space for one 18", let alone a pair!
12's and 10' are the best speakers to work with, they allow you to use multiple drivers, you have smaller enclosure spce that is required and the sound is just better. The fact of the matter is that you can achieve more sound and higher spl with multiple drivers of smaller size than of what you can get with just 1 or 2 huge drivers.
I think that you are just trying to put something in your car, and you have no idea of what you are doing. Go to some competitions in your area and talk to the guys, see what they are hitting and what they are doing it with. Go to your local shops, ask questions, obviously you have lots because you are fishing for whales in a puddle.
Finally, take some time to do some research!!! If you want your system to crank make sure you know what it is that you are doing!!! Four Yellow tops??? Uh- I don't think so. See you are already headed for disaster. Go to http://www.sounddomain.com
and start asking questions there. They have lots of different forums geared for all levels of knowledge, classes of sound, and $$$ to spend. You are in serious need of instruction and I along with lots of others are willing to help, but damn, look before you leap. I spent four months reading, listening in stores, asking questions all over and just learning all I could about the products I wanted before I even spent a dime. It will be to your advantage if you do the same, I knew what my system was going to sound like even before it was in, and now that it is in, I have no regrets, I hope only the same for you.