I have some spare 4ga wire left over after installing my amp and I want to add a second battery & switch. could I get away with using the 4ga wire to connect to the switch or do I need heavier gauge?
Yes, 1/0 will be good if the battery is being used for things other than the motor. Plus, if the batteries are close to each other - you will only need 1-2 feet of 1/0.
Just some food for thought. If the the house (stereo) bank will be connected to a dual battery switch and used to start the engine on occasions, keep in mind that a typical V8 engine starter can easily draw 250A.
I'm using 4awg for switch connections and 2awg for bank grounding connection and starting bank to starter connection with no issues. It all depends on your setups draw. Blue Sea has a sizing chart for awg and fuses.