Any four cell 4.8v pack Nicd or Nimh that is rated at around 600 Mah or more is perfectly fine for your airplane. That is the rating of the packs included with most new systems.

You can also use a five cell pack 6.0v but it will not buy you anything in that airplane or improve your learning experiance in any way.

Running the higher voltage will slightly improve the servos speed which at this stage you will not be able to tell the differance and using the five cell battery will also use more current so the battery will not last as long.

Thats why when using a five cell the capacity should be higher than than what you may use with four cells packs.

Also keep in mind when using five cell packs and higher capacitys to assure full and complete charges you are going as a practical matter need a more capable charger.

John