Don't get those ebay stick packs. Get ones like these: Ebay batts
Watch and see if you can get a couple in one auction. Last year I got two six cell packs for $23 shipped. They just like those and I still race with em. If you're not competitively racing they should do you fine even being unmatched.
Or you can usually find those kind of batts in the for sale forums here pretty much everyday.
And yes the 4x4 will eat more of your batt power as the motor is working harder to turn 4 tires instead of two. What motor do you run in it?
You may need a different charger to use these nimh batts though. What charger do you currently have?