Hi TTim: 3 gear and 4 gear both have there place depending on what you want to achieve. Personally I prefer 4 gear most of the time but this is mostly due to the tracks I usually run on. I run at OCRC and SCVRC here in Southern California and the surface is a nice high grip clay indoors. So traction is pretty consistent. As with anything it's best to try it to see how it feels to you and notice how it makes the car different.
A couple of very general notes about the differences in handling:
MM4: The car will respond to throttle and braking much more than with a 3 gear setup. This is due to the rotation of the wheels and transmission and there relationship in terms of direction. The car will transfer weight to the rear on throttle and on braking transfer weight to the front.
MM3: The car tends to drive flatter and respond much less to throttle and braking. Same reasoning here. The direction of torque on the motor versus the wheels and transmission, these are the two largest inertial bodies interacting with the car.
I've found this to be very helpful, especially the diagrams.
http://www.team-durango.com/blog/201...onfigurations/
I've found an easy way to see and understand this is to hold the car ready to run and give it throttle and brake and see how the car wants to nose up on throttle and nose down again when you hit the brakes. Switching to MM3 will lessen this effect. Hope that helps!