Final answer:
The maximum amount of CAT IV small arms that may be sent via registered mail with the U.S. Postal Service cannot be determined without current authoritative guidelines. Strict regulations apply to any firearms shipment, which include various documentation and packaging requirements.
Step-by-step explanation:
The question relates to the regulation surrounding mailing specific items, in this case, CAT IV small arms. The regulations for mailing weapons, including arms classified under Category IV (CAT IV), are stipulated by the U.S. Postal Service and other relevant laws. Unfortunately, without current authoritative guidelines or access to relevant legal and postal service directives, providing an exact answer on the maximum amount of CAT IV small arms that can be sent via registered mail is not possible. However, it is essential to understand that any shipment of firearms is subject to strict regulations that generally include documentation, packaging, and carrier requirements. For the most accurate information, one should consult the latest U.S. Postal Service regulations, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) guidelines, and applicable federal, state, and local laws.