[Djur]

2 CIMs are lighter than 4 (and their gearboxes), but they offer so much less power that many robots with 4 CIMs can bully 2-CIM robots very easily. While having 4 CIMs isn't strictly necessary -- you can easily drive a tank style bot with only two -- it removes the power inequality with only 2 CIMs and makes a more responsive/maneuverable/powerful robot.