I believe you have (2) 4" in the cabin and (2) 6" in the helm area. I used 4" and 6" Fusion speakers and they were a direct replacement, I could even use the wiring as the connectors were identical. I added (2) additional 6" in the steps going to the walkaround where the circular cockpit lights are located (I assume you have these, if not the placement is on the vertical step as you go up the walk around. I used an 6" hole saw to cut the holes and I relocated the cockpit lights so I still have these.
I went with the lower cost 2 channel amp from Fusion and a Fusion head unit in the cabin. The amp was placed just inside the panel on the port side in the aft cabin. The Fusion head is great and you can control all three zones independently. I have it quieter in the helm than the cockpit and even quieter in the cabin. Didn't bother with the sub, so I'm using only one of the 2 channels on the amp. The head unit powers the other (4) speakers. If I was doing a Sub, I'd look at the area under the aft seat, not sure what year you have so can't comment on your boat. Just get good marine equipment and you'll be fine.