Why do people have an obsession with subwoofers?????
imho,
1. throw out the 4 inch speakers. 4 inch speakers are good for nothing. Replace them with some decent quality 6 inch splits. This will make the biggest difference to your overall sound quality. That is if you are interested in quality. If all you want is distortion and doof doof, then keep it te way it is set up atm.
2. Amp up the splits in the front doors with the other two cannels of your existing amp. run the 6x9's off the head deck. Or alternately disconnect them altogether. I think you'll be surprised that less speakers sometimes sounds better.
basically what nav2k said.