Most of the braking effort is carried by the front brakes. So, I see little point in fitting massive brakes on the rear. In the worst case, you will get rear-end lockup every time you apply the brakes, due to the increased efficiency of the rears. As a best case, you risk having massive pedal travel, considering the difference in volume of the 2-pot pistons vs the 4-pot pistons (I'm guessing the 4-pots are larger displacement).