Comparing capacity on batteries depends on their amp hour capacity. You had 2 12 volt batteries connected in parallel. Connecting batteries in parallel keeps the voltage the same but doubles amp hours (AH) which is the capacity.
When you have 2, 6 volt batteries, you have to connect those in series, which doubles the voltage to 12 volts but keeps the AH the same. The best description I heard is when you connect 2 6 volt batteries together you are essentially making on big 12 volt battery. To connect 4, 6 volt batteries, you will be connecting 2 each in series and then connecting each pair in parallel.
This all being said, 6 volt batteries "generally" have more AH's than 12 volt batteries do. Not a bunch more but more. So, generally, 2, 6 volt batteries will will have more capacity than one 12 volt battery. Now, 2, 6 volt batteries may or may not have more capacity than 2, 12 volt batteries. Again, this depends on the size of the 12 volt battery.
4, 6 volt batteries will definitely have more capacity than 2, 12 volt batteries. At the end of the day it all depends on they type of camping you do. Lots of dry camping or boondocking? 2, 6 volts at a minimum. Always camp at a campground with electrical hookups? Batteries don't matter as much.
Dry camp for long periods of time? You want as many batteries as possible.
Looking to use an inverter to change the 12 volts to 120 volts so you can use the microwave while dry camping? Lots of battery capacity.
My camper doesn't currently hold 4 batteries but due to our boondocking and dry camping, I am in the planning stages on modifications to hold 4, 6 volt batteries.