Massachusetts law is written without any regard for federal licensing, dealer or C&R. As a result, it tends to overlook situations that would be perfectly legal for a licensed person under federal law, but illegal for unlicensed individuals. As a result of Simon's Rock College shooting (late 80's) the legislature prohibited sales of long guns (otherwise allowed under federal law) by Massachusetts dealers to non-residents. However, there's nothing in the law regulating interstate sales by those who are not Massachusetts licensed dealers. The 4 sales per year limit is explicitly written to apply only to sales by one FID/LTC holder to another. As a result it applies to sales by one C&R holding resident to another, but not by a C&R holding resident to a C&R holder in another state. (At least that's the plain language of the law. It wouldn't surprise me in the least if the AG's office were to consider such transactions patently illegal, despite the total absence on any statute or regulation prohibiting them.)
Ken