Article Four of the United States Constitution

Article Four of the United States Constitution outlines the relationship between the various states, as well as the relationship between each state and the United States federal government. It also empowers Congress to admit new states and administer the territories and other federal lands.

The Full Faith and Credit Clause requires states to extend "full faith and credit" to the public acts, records, and court proceedings of other states. The Supreme Court has held that this clause prevents states from reopening cases that have been conclusively decided by the courts of another state. The Privileges and Immunities Clause requires interstate protection of "privileges and immunities," preventing each state from treating citizens of other states in a discriminatory manner. The Extradition Clause requires that fugitives from justice be extradited on the demand of executive authority of the state from which they flee. Since the 1987 case of Puerto Rico v. Branstad, federal courts may also use the Extradition Clause to require the extradition of fugitives. The Fugitive Slave Clause requires the return of fugitive slaves; this clause has not been repealed, but it was rendered moot by the Thirteenth Amendment, which abolished slavery, except in the prison system.

The Admissions Clause grants Congress the authority to admit new states but forbids the creation of new states from parts of existing states without the consent of the affected states. The Supreme Court has held that the Constitution requires all states to be admitted on an equal footing, though the Admissions Clause does not expressly include this requirement. The Property Clause grants Congress the power to make laws for the territories and other federal lands. The Guarantee Clause mandates that the United States guarantee that all states have a "republican form of government," though it does not define this term. Article Four also requires the United States to protect each state from invasion, and, at the request of a state, from "domestic violence."