do you have sync? if so then you can get a harness that will retain sync features. however the sat is built in to the factory headunit and can't be kept if you go aftermarket. you will need another module for the brand of headunit you use. you might be able to reuse the factory sat antenna though to keep things looking clean. I was able to on mine.
if you want to do it the way you mentioned then there are a couple options. you can just tap on to get signal for the 4ch (don't cut speaker wires) and then disconnect the factory wires from the speaker. you will then need to run new speaker wires from the amp to the speakers.
the other option is to cut the factory wires and use those for the amp outputs and move your sub loc up to the front with the 4ch one.
2010 Volvo C70
2011 Harley FLHTK
Midwest Crew #22