I sold my M4 because I bought an M6; I felt the M4 was just a stepping stone to the M6. The M4 has a beautiful, strong beam but it was too long with 4 cells in series (it would not fit in my pocket) it was a shade longer than a 2C Maglite.
I will be trying to decide how to use the M6 with Li-Ion cells since I don't want to be feeding it primaries. I like that I can use 2 or 3 cells beside each other rather than end-to-end. I just have to decide on which bulb to use for about an hour of runtime.