Al,
I agree that the KL5 should not be hosted on 3x123. However I believe that hosting the KL6 on a M4 is a safe combination. It is my understanding that the Kl6 can handle a Vin above that which you would see from any 4x123 cells. I have a KL6 on M4 and like the feel and balance of the light. For momentary on, the weight is not balanced well with all of that light out there. However, for a walk around, in constant on, I think the KL6/M4 is an ideal package in terms of thermal relief and battery capacity. The KL6 I modded with a Luxeon III is even a cooler and longer burner with greater reach. IMHO, this combo provides a very useful tool at the other end of the momentary/ tactical applications.