Well I have no idea of run time due to an attention span defficiency but the Kl6 is fine on the M4 to the best of my knowledge. I asked PK about the KL6 and he said that 4x123 was OK.
I just tested a KL6 I have by measuring the current when the amp meter probes completed the circuit between the rear battery and the battery tube at the tail (tail switch removed).
On the M3, the current drawn was 690 mA. On the M4, the current drawn was 548 mA.
EDIT: I might, in good conscience, add a cautionary note here! A while ago, I played with a KL6 powered off of a bench power supply. After accidentally shorting the two springs while the KL6 was lit up, the PCB was ruined somehow! From that point on, the LED was in DD. This wasn't a fluke as my pal Roger did the same thing to his KL6. /ubbthreads/images/graemlins/icon15.gif Fortunately a DB1000 and Lux III were easy to install and I could say that I meant to do that! /ubbthreads/images/graemlins/tongue.gif