In this paper, a novel technique to reduce the leakage current under an assigned delay constraint is presented for FinFET forced stacks. This technique is based on the adoption of different back bias voltages in stacked four terminal (4T) FinFETs (as is well known, this would not be possible in bulk CMOS circuits). In particular, a Reverse Back Bias (RBB) voltage is applied to one of the two stacked transistors to reduce its leakage at the cost of a delay penalty, whereas a Forward Back Bias (FBB) voltage is applied to the other one to compensate this delay degradation. Mixed device circuit simulations for 40-nm FinFETs show that the proposed "mixed FBB/RBB" technique permits a leakage reduction by one order of magnitude or more as compared with traditional transistor stacks at same delay.