You don't need 4 batteries. I been a dry camper for 40+ yrs, and have never seen a need for more than 2 batteries.
Remember 4 discharged batteries is much worse than 2 discharged batteries.
And whats better, 2 hrs gen run time on 2 batts per day, or 4 hrs gen run time on 4 batts every other day ?
Now if you have to depend on solar power, that can be a different story when you want to store as many amp hrs as possible for those days when the sun don't shine.
Sorry, didn't answer your question, to which the answer is it will probably ok to just add 2 batts, if the 2 you have are good.