interestingly enough, mine has much longer runtimes. I'm not using a zzmt motor though, I use a motor out of a clone that is slightly bigger and has lots of torque.

Motors will draw the most current when under heavy load. It's true the motor has to turn more to move the truck, but in either gear setup, the motor would reach its peak rpm. In the 4 gear case, at peak rpm, the load would be less. If you're really working the load to crawl over things, then the battery life will go down.