Hi Chris,
Welcome to CPF! /ubbthreads/images/graemlins/wink2.gif
There are users who will use two Pila 168S Li-ion batteries in place of the 4x123's. There is a voltage difference (7.4v versus the 12 of 4x123's). I have been considering trying this configuration myself. I have not tried any rechargables in my M4 yet, perhaps another member of the forum can help with the difference in performance.
There is also an evaluation if a Li-ion 123 rechargables going on as we speak.