Which article explains how each state should relate to other states and the federal government?

"Article IV" is the right answer.

The Federal system of government in the United States distributes the power between a National government and the government of States. The constitution of the United States gave certain power to the national, other power to the state and yet other powers to both. Therefore, Article IV of the Constitution sets the obligations of the states to each other and the duties of the federal administration to the states.