"Is it better to fit four small 6V batteries, or two large 6V batteries?"
Check the Amp Hour ratings of the batteries. With 2 large batteries the total AH will be the AH of one battery. With 4 small batteries the total AH will be double the AH of one battery. I believe you will have the same problem with the series parallel battery arrangement as you would have with parallel 12v batteries. Maybe someone with better knowledge would correct me if I am wrong.
When comparing AH ratings compare apples to apples. The standard AH rating is a 20 hour rating. Some sellers may use a different hour rating that makes their battery appear better.
