Found this old thread as I plan to bore out a brand new M4 (as soon as one is snagged on Buy-Sell-Trade or on eBay).
The problem mentioned by those that did the work was that the o-ring grooves at the tailcap make the final product weaker there because of the decreased thickness.
I strongly believe that is a non issue. Measure any "standard" Surefire tube, meaning those tubes that have an outside diameter of 1". I've measured dozens, including 3P, 6P, 9P, C2, C3, M2, M3, M4 - all measure in the range of .999" to 1.002", showing that the company holds a pretty tight tolerance on OD.
The area of concern is the outside diameter at the O-ring groove. Measuring all the lights mentioned above shows measurements in the range of .774" - .776"
To fit most 18650's, the inside diameter of the tube needs to be at least .734". To fit every 18650 that I've been able to measure, the ID needs to be slightly larger, at .738". Modders have been boring Surefire bodies out to this dimension (.734"-.738") for years, and I have yet to hear of a single light that failed at the O-ring groove. The walls are still substantially thick (.775" minus .738" leaves .037", divided by two = nearly .019" wall thickness).
In comparison, I've found thinner walls in commercially available tubes. My FiveMega tube (for the 9P) measures .775" OD at the O-ring
and has an ID of .750". This leaves the walls about .013" thick, quite a bit less than a bored Surefire, but still practically indestructible.
My current "experimental project" is to bore my personal E2L for 18650. The E-series lights ARE very small to start with, but I believe they can be bored to very closely fit an 18650 and still leave about .013" wall thickness. All experiments are first performed on my personal lights, before this service is offered to the public.
The walls are easily thick enough for boring,
That's certainly the way I see it. The M4 should present no more challenge than an M3 or M2.