Article Four of the US Constitution outlines relations among the states, and between each state and the federal government. It sets out the responsibilities and duties of the states as well as responsibilities that the federal government has to the states.
Article Four sets the rules for such matters as admitting new states and border changes between the states. For instance, the first Section, called “the Full Faith and Credit Clause,” stipulates that each state must recognize and respect the public acts and decisions of other states. It also stresses that the Congress may monitor such interactions between states, and may stipulate how proof of such acts may be admitted.
The "Privileges and Immunities Clause” also prohibits state governments from discriminating against citizens of other states.For example, should two people, one from Michigan and the other from Ohio, be convicted of the same crime in Michigan, the state may not impose a tougher penalty on the resident of Ohio.
Article Four also provides for extradition of fugitives between the states, and lays down a legal basis for freedom of interstate travel. Today, this provision is sometimes taken for granted, but in the days before the Constitution was written, crossing state lines was often difficult and costly.
Under Article Four, the federal government of the United States guarantees to every state a democratic form of government, and will protect each of the states against foreign invasion. If a state’s legislature, or its executive (when the legislature cannot be convened) asks for help, the federal government is required to defend it against domestic rebellion or political violence. As well, Article Four empowers the Congress to make rules for disposing of federal property and governing non-state territories of the United States.
In Federalist No. 43, James Madison wrote: “A protection against invasion is due from every society to the parts composing it. The latitude of the expression here used, seems to secure each state not only against foreign hostility, but against ambitious or vindictive enterprises of its more powerful neighbors. The history both of ancient and modern confederacies, proves that the weaker members of the Union ought not to be insensible to the policy of this article.”
In other words, the Constitution protects all states equally and guarantees to the citizens of each state a democratic form of government.