Article IV of the U.S. Constitution is a rather short one with only four brief sections. It covers certain state issues, such as what needs to happen for a state to be a state, how citizens are treated as they move from state to state and whether fleeing a state frees people from certain obligations. There is a clause that is now null thanks to future developments in the U.S., but it will be discussed here anyway.
To read Article IV in its entirety, click here.
Section 1 of Article IV is pretty straightforward. All states are to give "full faith and credit" to "public acts, records and judicial proceedings" of all other states. Section 2 states in so many words that travelers into states who are citizens of another state have the same rights and privileges of that state's citizenry. Also, criminals who flee a state into another may be brought back to face justice.
Section 2 contains an interesting part of Article IV. This clause is something akin to the Fugitive Slave Act that would come much later. In essence, it states that people held in "service or labor" in one state cannot flee into another to escape bondage or owed labor. They can lawfully be claimed by those who keep them in service and returned from whence they came. This was rendered null by the 13th Amendment.
Section 3 of Article IV involves admittance of a state into the Union. Only Congress *and* the states concerned can combine states to create a new state or create a state within a state's borders. As for new states that do not fall within existing states, only Congress has the authority to admit one. Moreover, Congress can make "rules and regulations" about U.S. property, particularly land.
Finally, Section 4 of Article IV involves the government of each state and federal protection. Each state is to have a "republican form of government." The federal government must see to this. The federal government also must protect each state from invasion and "domestic violence."