[MrForbes]

Maximum top speed you can get should be higher with two motors, since you have twice the power available, but teams usually set up the gearing to limit top speed. There are several reasons for this, including being able to control the robot, and having enough pushing power at low speeds (which you lose when you gear the robot to go faster). A good compromise might be to have a two speed transmission, so you can have good pushing power and still have the robot geared for a fast top speed.

Getting up to top speed will be faster (quicker acceleration) with 4 motors.