While I don't have a good cite on hand, my understanding was that firearms imported for law enforcement sales are exempted from 922(r).
If the Benelli M4, as configured, is unsuitable for sporting purposes, then it must have been imported for LE sales and purchased under the authority of the police department (either as an individual officer purchase, or the officer bought it from the department at a later time).
18 U.S. Code § 922
(r) It shall be unlawful for any person to assemble from imported parts any semiautomatic rifle or any shotgun which is identical to any rifle or shotgun prohibited from importation under section 925 (d)(3) of this chapter as not being particularly suitable for or readily adaptable to sporting purposes except that this subsection shall not apply to—
(1) the assembly of any such rifle or shotgun for sale or distribution by a licensed manufacturer to the United States or any department or agency thereof or to any State or any department, agency, or political subdivision thereof; or
(2) the assembly of any such rifle or shotgun for the purposes of testing or experimentation authorized by the Attorney General.
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18 U.S. Code § 925
(1) The provisions of this chapter, except for sections 922 (d)(9) and 922 (g)(9) and provisions relating to firearms subject to the prohibitions of section 922 (p), shall not apply with respect to the transportation, shipment, receipt, possession, or importation of any firearm or ammunition imported for, sold or shipped to, or issued for the use of, the United States or any department or agency thereof or any State or any department, agency, or political subdivision thereof.
....
....
(d) The Attorney General shall authorize a firearm or ammunition to be imported or brought into the United States or any possession thereof if the firearm or ammunition—
(3) is of a type that does not fall within the definition of a firearm as defined in section 5845(a) of the Internal Revenue Code of 1986 and is generally recognized as particularly suitable for or readily adaptable to sporting purposes, excluding surplus military firearms, except in any case where the Attorney General has not authorized the importation of the firearm pursuant to this paragraph, it shall be unlawful to import any frame, receiver, or barrel of such firearm which would be prohibited if assembled; or
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While I hate to link to another board, there's an ATF letter here concerning reassembly after cleaning:
http://www.falfiles.com/forums/showthread.php?t=261731
Per that letter, disassembly/reassembly is ok as long as you're not replacing parts.
http://www.law.cornell.edu/uscode/text/18/922
http://www.law.cornell.edu/uscode/text/18/925
YMMV and all, but I'd go for it. The Benelli M4 is an awesome shotgun.