You need to have 3 sets of RCA's going from your 420 to the amps. 1 set will power the sub. That's easy, it's the sub channel on the 4 channel amp. I'm pretty sure that the 4 remaining channels of that amp are powering your cabin/bow speakers. The 4 cabins will probably be on one pair of channels at 2 ohms, and the bow speakers are going to be the remaining 2 channels at 4 ohms. Since you're only going to run a single set of RCA's to the Bow/Cabin channels of that amp, you'll want to make sure you press the switch to sum the inputs of that single RCA to the 4 channels of the amp.

The last amp would be the tower speakers and it'll get a pair of RCA's from the 420. It's Possible that there's a set of passthrough RCA's going from the 5 channel amp to give an input to the tower amp, but that would kinda surprise me. Bottom line is that you'll have 3 pairs of RCA's leaving the 420 and they need to go to Sub/Cabin/Towers. You'll also have a SINGLE set of RCA's going from your headunit to the 420, so no more fading from the deck itself. (that's why you bought the zone controller in the first place.)