I'm not sure I follow. I've never looked at Form 4 prior to now but in glancing at it I'm not sure what basis the ATF would have for rejecting me. Could you elaborate?
Welcome to the world of NFA.
You would be rejected because an M4 is manufactured after 1986, the cutoff date for federal law 922(o) banning civilian ownership of machineguns (pre-’86 MGs are grandfathered). Form 4 is rejected for anything you are not allowed to have.
The interesting bit is that the M4 is exactly what the Founding Fathers had in mind when enacting the 2nd Amendment (and emphasized by the Militia Act of 1792): unhindered (nay, encouraged) civilian ownership of standard/common/mundane modern military arms. You, a retired military man and now explicit member of the unorganized militia, with presumably clean record and upstanding citizen, should constitute a perfect example of the intention of the 2nd Amendment that you should, without infringement, be able to own a new M4 for defense of home and country without infringement (and we’ll even overlook the Form 4’s “infringement” of no possession until registration & $200 tax) - but 922(o) forbids you from doing so, and invariably causes a Form 4 rejection.
Make sense? (I’d do it if I had the money to spare. You have the advantage of honorable discharge from military service.)