Well, it's totally your call. A 5-cell pack is not NEEDED for your application, and you SHOULD get a charger designed for charging 5 cells (if you don't already have one).

One thing you didn't mention is your radio. If you are using 72MHz or Futaba 2.4GHz, 4-cell packs are no problem. But if you're using JR or Spektrum 2.4GHz, the 6-cell pack is cheap insurance against brown outs.