If your going with four batteries I would recommend you got with 6 volt deep cycle batteries. You will get greater amp hours with four 6 volts vs 4 12 volts. About 440ah with 6 volt and 260ah with 12 volts. That said when I did this I went with 12 volt deep cycle batteries because my trailer was new and I only had to buy 3 additional 12 volt batteries and when I bought my deep cycle batteries at Costco the 12 volts had a 2 year warranty and the 6 volts only had a 1 year warranty. I don't know if that's still the case or not.
You got some reading to do. 6 volts are wired in series where 12 volts are wired parallel.
So, I would get your batteries at Costco
Some reading to do so you can make a informed decision.
http://www.batteriesnorthwest.com/batteryschool.cfm?TID=6
