Articles One, Two, and Three of the Constitution divide and separate national power horizontally. What Article Four does is, as in effect, to recognize the independence of the states and divide power with the national government sort of vertically. For example, Article Four requires states, each state, to give full faith and credit to the judgments of the other states. Now what this means of course is that each state has its own system of courts and its own domestic law. And under those laws it will reach judgements and grant divorces and find people who are owners or not owners and so forth. And the judgments that each state courts reaches must be recognized and respected by courts of differing states even though those other states might have reached a different judgment under their particular laws. Article Four also protects the privileges and requires states to acknowledge and protect the privileges and immunities of the citizens of the country. Now what that means, it's a little hard to get a good fix on privileges and immunities, but it essentially means that you have a right to travel among the states, without any difficulty getting across the border. And the state you go to has to treat you just like they treat their own citizens Article Four, in addition, also says that the national government will protect states from invasion by a foreign territory, by a foreign country. It guarantees to each state a republican form of government. Which means that even if the states decide they want their own king, their own monarch, who's going to be a hereditary monarch, the federal government can prevent that because it requires that majority rule be what happens in every state. So the great thing about what Article Four does is it offers national protection to the states by protecting them, for example, against foreign invasion, but it also permits them to and permits the citizens of the various states to have considerable freedom of movement. It permits communication. It permits travel. It permits commerce and so forth among the several states without any barriers and without any inhibitions.