Actually, I think that EADS had planned the A-310 MRTT with 5 tanks and modified it to 4 tanks for A-310-304 use. On the EADS website:
"The A310 MRTT, with up to five additional fuel tanks in its lower deck, can be provided with a combination of wing pod and/or fuselage hose and drogue units and/or the Airbus Military Air refuelling boom system (ARBS)."
Airbus MRTT - Air to Air Refuelling Tanks