How would one set up 4 batteries in the Vanagon? I have a battery switch, with a type of relay between them, so one is the engine battery and one is the house battery. There is also an inverter that is wired only to the house battery. The battery switch allows for starting the engine from either battery. 

The idea of adding two more batteries to this confuses me. Would I need three battery switches, one two switch between the two other switches?... but I also want batteries three and four to run the inverter as well...