Article 4 of the United States Constitution deals with the relationships among states and between states and the federal government. It includes several key provisions:

• The Full Faith and Credit Clause mandates that states must respect the public acts, records, and court decisions of other states.
• The Privileges and Immunities Clause guarantees that citizens of each state receive the same privileges and immunities in other states.
• The Extradition Clause requires that fugitives who flee from one state to another be returned upon request.
• The Admissions Clause grants Congress the power to admit new states and regulate federal territories but prohibits creating a new state from an existing one without consent.
• The Guarantee Clause requires the United States to guarantee every state a republican form of government and to protect them from invasion and domestic violence, upon request of the state legislature or executive.

Thus, Article 4 primarily governs state interactions, the admission of states, and federal obligations to states regarding governance and protection.

There is also an unrelated "Article 4" in NATO's treaty, which allows member states to request consultations if their security is threatened but does not require military action. This is different from the Article 4 of the US Constitution.