Audio Systems: sub enclosure and woofer size, inch woofers, ohm load

Question
I have a 1981 toyota supra its a little 2 door hatchback I have recently installed 4 - 15 inch woofers single voicecoil 4 ohms each so i could acheive 1 ohm on my mono amp 2000 watts why is this set up not as loud as 2 of these woofers in a much bigger car my station wagon? and is it too much cone area in this small of car or what i am really knoweldgable in car audio but this really stumps me why it isnt louder I dont understand why my stationwagon is louder when all i read about sound says closer to the source is louder and also i made my amp put  out more power by dropping the ohm load to 1 ohm and i doubled the cone area at the same time any help would be greatly appreciated  

Answer
firs off NICE CAR!!!  
well, if 1 speaker is out of phase, it will phase out another speaker, which in the end will sound just like there's 2 speakers not 4.  combine that with the fact that 4 speakers is a greater load than 2 speakers, which means the remaining 2 speakers would have less power than 2 alone on that amp.  
Basically what I'm trying to say is that all 4 subs need to push and pull in unison.  if even 1 speaker is stroking in the oposite direction it will have detrimental effects on the output.
To easily test this,  disconnect the speaker wires at the amp.  you said its a mono config so you should only have 1 "set" of speaker wires at the amp.  once disconected, place a 9 volt battery across them and all 4 speakers should move in the same direction.  (doesnt matter which, either push out, or pull in), but they ALL need to move in the same direction or it will sound like crap.  if one or more speakers moves different from the rest, switch its wires around and try again.  you need them to all move together the same way and then you'll have a bangin' system (and car!)
Let me know how it goes, if that's not it we'll move from there.