I suggest you return it and get a Mesa Tactical 8 shot carrier with intergrated top rail. I find Mesa to be built stronger and are not so boxy. Mesa is also very easy to install and look awesome on the M4. Just my two cents.
I will suggest buying the Mesa 8-shot shell carrier for the High-tube Remington 870 adapter, and cutting the small nub on the end off. Then use some outdoor rated double sided high strength tape to attach it to the side of the receiver.
Not only is it stronger (you can put your fingers through the loops and shake the gun as HARD as you can, and it will stay attached), but lighter and less likely to damage the gun if it gets hit. After a 5-day hunting trip with buddies and the gun being wet and in crappy conditions, its still holding firm.
I got the 8-shot Mesa shell carrier for my Benelli M4, and tightened it to the recommended 18-20 inch pounds. After each shooting trip it was quite loose, so I would re-tighten it to the suggested torque (I did use loc tight BTW).
After I took it off to see what the deal was, I only had about 50% of the threads left in my receiver
.....
There was just enough thread left for me to use a high strength epoxy to attach the stock rail back on....
This is the guy you want:
(It is the least expensive 8-shot carrier I could find)
To do this:
ETA
The M4 shell carrier rail destroying the receiver threads is a well known problem....
