first off, welcome to the forums!
second, as your first post is about car audio, this forum is mainly for car computers and integrating them with the cars audio system, so you might get a different/better/faster answer from a dedicated car audio forum(sticky at top of this forum topic)
to answer your original question--usually for a setup like that, you wuold run 2 sets of rcas back to the amps, and leave any crossover settings on the radio off, and setup the amps to crossover all the speakers.
the 4 channel amp should have a output line, this would need to be used to be connected to the sub amp.
set all the crossovers and gains on the amps to their respective points, and your good to go.