When you say whole system that includes the sub for the 5 channel amp? I'm clueless about this kind of thing but hope to learn soon when I get help installing from a buddy.
Could I use the 5 channel to run the head unit and speakers and use the 4 channel to run the sub or something in that combination?
Alright lemme do a little explaining haha
Head unit doesn't need power from amp, it only sends signals to the amp to tell the amp what power to give to the speakers, and the volume and frequency etc. the head unit gets its power from the battery through the wiring harness
Now in the case of a 5 channel you have 4 pathways (channels), 1 for each speaker, that are each equally powered (40 watts). Along with 1 extra channel, made for a subwoofer that has 300 watts. So you can run a good stereo off just the 5 channel but your door speakers would not be nearly as loud as if you were to use the 400 watt 4 channel amp. This is because that amp puts roughly 100 watts to each speaker. Depending on what kicker speakers you got that's probably way too much power anyways and you'll blow them.
So in my opinion use the 5 channel amp, sell the 4 channel amp or keep it either way, but the 5 Channel will power all of your speakers including the sub with most likely plenty of power
RCA cables will give the signal to the amp from the back of the head unit telling it what to do you will need 3 sets of RCA cables sometimes called signal cables they have them on crutch field. You said you already had wires so you may ready have them.
I can probably answer any question you have about stereos but it took me a lot of research haha so don't feel bad about not knowing it took me months before i started to pick up on things