yep, with a 4000kv motor and stock gearing you are trying to move double the weight intended. If you want to stick with the 4000kv motor, you will need to run a much small pinion (motor) gear. Not sure how small a gear can fit, but something as small as a 16tooth might work.
You will have the same issues if you use the stock 20t brushed motor. If you don't mind the truck being slow, you could buy a rock crawler brushed motor (which are cheap) A 35 turn brushed motor might be enough...maybe even a 50 turn. A higher turn motor has more torque but less rpm's. But you might be able to run a crawler motor with the stock gearing.