The wire connection is at the circuit breaker is probably not suited for a continuous load. Boat stereos sustain much higher current than car stereos, most shops don't understand the difference. #4 wire is probably too small for the runs between the battery and the amps if the run is over 4'. Sure it will sustain the current but it will drop the voltage, made the same mistake myself first time around and I am an electrical engineer. You should be running 1/0 cable between the batteries and the amp distribution block.
Here is a picture of my fuse block, it's a $200 unit once you buy the fuses. It will run full current all day long without even getting warm.
(Message edited by mikeski on June 17, 2008)