Id like to use a marine radio/cd player to power my 4 outside rock speakers. They are 8ohms but the marine stereo (waterproof) puts out 4ohms. Will this work and if not, how can I incr from 4 to 8ohms easily ?
What do you mean the stereo puts out four Ohms? Amplifiers don't "put out" Ohms. Amplifiers will normally be safe into a higher impedance load but will have reduced power output (watts).
I should ask how you are running the speakers off the stereo. How? If they are connected in parallel and directly connected to the amplifier outputs (even through a simple switch), they will essentially half their nominal impedance. If you run through an autoformer type switching box, you will maintain a constant load of your choosing.