You don't want to use cells in parallel. I won't get into the technical reasons but suffice it to say - not a good idea in a battery pack. Stick with 4 cells but get cells with larger capacity. I'd say try NiMH of larger capacity in each cell.
This http://www3.towerhobbies.com/cgi-bin...&I=LXNRH6&P=ML is just one of many available and don't limit your search to Tower or any other single dealer.

Remember that bigger 4.8 volt packs will take longer to charge and 6 volt packs would definitely require a different charger.