The CC MMM Combo runs really well on 4S lipo. Use a pr of 2S lipos and wire them in series for a 4S combined pac. The pr of batts will also balance better on the chassis than a single 4S will. They still have to fit the batt compartments and pretty sure 2S 5000mah will fit with no probs.
The cost of lipo is partly driven by the cell quality and any matching thats doe when they are assembled. Enerland and Kokam are prob teh best qualit lipo out there and they cost a little more up front. But they perform better, run longer, stay in balance longer and stand up to abuse (hopefully accidental) better than the cheaper quality cells do. SOme of the lipo coming from China do amazingly well for their cost (Zippys from HobbyCity), but I try yo stay with namebrand ones. I have a pr of NeuEnergy 2S 5000mah and they've been awesome.
This is a pretty good FAq on E-Revo and its worth a few min to provide some background and maybe answer some ques -