Okay I think this may help if I'm reading correctly you only have 1 sub and if from your description of the way its connected to your 4 channel amp it is wired incorrectly. You should be able to follow the diagram below courtesy of SoundDomain and correct your issue.
With your amp I'd bridge 2 channels as seen below from the manual for your amp
Also make sure your ground wire is the same gauge or larger than your power I believe you said 4 agw. When you connected your ground to the chasis of your vehicle did you grind off paint to bare metal? If not you will want to do so.
Heres a link to Sound Domain wiring wizard really usefull