Interesting. I wasn't aware that they changed the law. M4's are not sold or resold to civilians, even military personal and law enforcement. The military discontinued allowing war-fighting arms to be retained off base in 1993 a la Clinton. All private citizens are prohibited from personally owning fully automatic weapons manufactured after 1986 a la Reagan. Non-military governmental agencies may obtain and issue to their employees, but retain ownership and estate custody. It must have been an interesting purchase.

ATF Online - Firearms - National Firearms Act (NFA)

A 1928 Thompson runs about $17,000 for one in average to OK condition. That's quite an impressive acquisition. Good luck.