4000MAH15C is marginal. For branded battery (such as Team Orion) should be ok. For China local one, the capacity or the C may not have as it is rated. Try to use higher gear ratio la.
He he, I use this formula. If the car can be played for 10mins the motor A電 is 60 / 10 x 4000mAH = 24A (average current). Then you would say 15C is 60A which should be more than enough. 24A is average but peak current can be double or more. 15C is to handle the peak current, not average. Some Li poly battery have separate peak current rating.