mickey d; I actually misspoke when I said that I have the QD cup in my M4 stock. I DO have it installed in my 590A1 stock but, I used the NOVESKE replacement sling plate on my M4 that has a QD cup on one side of it.
But as I said, if you opt for the cup, it will work, the threads are long enough. The 590A1 stock has thicker walls than the M4 stock.
I think the sling plate is the better option for the M4 unless you have the collapsable stock. It just takes a little work to replace the OEM plate with the NOVESKE plate.
Sorry for the confusion