[DRIFT_BUGGY]

Well the MM is rated at 6-12 NiCad/NiMH or2-3s LiPo so you are doubling it's limit. You will need to run a BEC, aren't you using a BEC with the 4S?
A Battery Eliminator Circuit is a little device that eliminates the need for a receiver and servo battery pack. It draws higher voltage from the motor batteries and drops it to a voltage level that is suitable for your receiver and servos. This is required in applications which draw high power for multiple servos or use more than 3S motor packs, as most ESCs with linear BECs are not designed for these applications.