personally i would never buy power acousik, but everyone likes what they like. but if you have them you can wire them at 4ohm, but from what i hear power acousik amps are overrated so you'd be underpowering the subs. 4 gauge should work fine for the wiring. and a 3 farad capacitor is way to big for that, the most you'd need with that set up is 1.5, but i would recommend not hooking the cap. up.
depends on if your alt. is big enough. adding a cap. will add in the dimming of the lights, etc. but with that set up you probably wont see much of a difference with or without it. and without upgrading your alt. your only going to put more of a strain on it.