Interpreting the federal governments role, why should article 6 override article 4?
Because of your tag, I am assuming that you are asking why the federal government should be able to have something like the "Defense of Marriage Act" that can override the idea that states have to honor same sex marriages that are legal in other states.
The general idea here is that any law that is legal for the Congress to make takes precedence over any law made by a state. So, as long as the DOMA is legal, then it must override state laws on that subject. Of course, a judge has recently ruled that DOMA is not a legal law. We'll have to see what happens with that next.
Philosophically, the reason why federal law should override state laws is that we want to have people of all states having the same basic rights. We are supposed to be citizens of the United States first and citizens of our individual states second. If that is the case, then the federal government should be able to ensure that marriage will mean the same thing in every state -- there should be no important differences in state laws on important things such as marriage.