I've said this a bunch of times, but never really took pictures or explained my position until just now, when I was prompted by another thread to show exactly what the problem is.
The problem is that the m4 rail is slightly "shorter" than a 1913 rail when you measure it's height from the receiver, but more importantly it is made up of "stubs" instead of a solid "bar" of rail.
The result is that optics mounts (most all of which are made for 1913 picatinny spec) have 2 small, shallow "stubs" to grab onto in order to anchor the optic, as opposed to a full height, full width "bar" that lets it fully grab on as was intended.
The end result of this is that your optic is not mounted to the receiver rail nearly as strongly as it would be the m4 used a proper 1913 rail. You can and will twist/scrap/contort/ruin your mount/rail if you use your weapon roughly, shoot heavy rounds consistently or drop/snag/smash your m4. It happens, rail damage occurs as a result in most cases.
I personally botched up up a Larue Tactical EOTech mount a few years ago in this manner, as I did not realize that the m4's bootleg rail would cause an issue. I shot my weapon quite a bit and handled it roughly, and I have an optic mount that has been "pushed" out of shape as a result. You can see it in the last picture ... other members on this forum and on other boards have had similar things occur.
As I was saying in the other thread, this is reason alone to consider getting a replacement top optic rail for your m4 ... but upon further inspection it seems that both Mesa Tactical and Sidearmor (the 2 companies I was going to suggest) both use the Benelli "stub" style rail, and they both claim it to be 1913 picatinny ... which it is not.
Behold, the 1913 picatinny specification:
As you can see, the spec calls for full "bars" of rail, not "stubs" as Benelli and all the benelli accessory makers seem to be copying.