Right, so if I understood your initial post correctly, you're sorted sub and sub amp wise, yes?
As for the rest of your speakers...
The choice is entirely yours - you can either use a four channel amp, or 2 x 2 channel amps.
Depends on what you can lay your hands on and what your budget is. I personally would go for 2 x 2 channel amps - allows for easier upgrade path; you can upgrade fronts + front amp, without spending money on rears etc. Your available choice for 2 channel amps will be much greater also.
You will need to run all new speaker cables from each of your amplifiers to the speakers.
I would advise that you unplug the 'speaker' output half of the ISO plug and leave it dangling behind the dash - this means you can leave the standard speaker wiring in place (so you can return the car to standard when you sell on) without the chance of any shorting.
