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force to the floor
The most important thing to think about is traction to the floor. It is important that whatever comes into contact with the floor be driven by a motor.
Let's say you design your robot to be two wheel drive and the coefficient of friction between your wheels and the floor is 1. If you distribute the weight of your robot equally, the most pushing force you can have is 65 lb (130 / 2 since half of your weight is being supported by wheels that aren't driven).
If you begin to push something such that the two driven wheels are away from the object your pushing, you might end up with all 130 lb on the two drive wheels giving you 130 lb of pushing force.
However, if you push an object such that the two driven wheels are toward the object you're pushing, you might take all of the weight off of the driven wheels giving you no pushing force at all.
Now let's say that all of your wheels are driven by motors (if you have 4, 6, 8 wheels - it doesn't matter). Then, no matter what your weight distribution is, you always get the 130 lb of your robot being supported by driven wheels, which means you'll always have 130 lb of pushing force.
So, to answer you original question, 4 is better than 2 from a standpoint of pushing force. However, the real answer is that whatever touches the ground should be powered by a motor.
-Chris
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