Great for CompM2 but caveat on CompM4
This is a typical high quality Aimpoint product. This 30mm lens cover will fit both the CompM2 and CompM4 and functions well on both. The only problem is that the fit is very snug; I recommend heating it up with very hot water before trying to install it. The only issue came when I decided to replace the alkaline battery with a silver oxide one and discovered that on the CompM4 the battery compartment cover will not clear the body of the lens cover and cannot be removed unless the lens cover is first removed. Installing the lens cover was hard, but removing it was really difficult. I wound up having to run very hot water across it to make it flexible, then use a plastic glass installing tool to get underneath it and pop it out of the groove. (Note: If you ever need to remove the lenscover, don't use something metal like a screwdriver or you'll gouge the Aimpont's aluminum body.) But battery life is such that this isn't something you'd have to do very often. Aside from that specific issue, I'd say the lens cover will do its job of protecting your optic's glass from the elements and most routine banging around.
