Ok guys. Once more, after doing research, calling and speaking with a number of Chicago Police Officers who are familiar with Chicago firearm laws said the M4 is legal. I described the M4 by saying that is is a semi-automatic shotgun and that the M4 has a pistol grip with stock and is 18.5" in length. CPD said it was legal. Based on that, I bought my M4 and I will update you after I register it.