[robertpaulsen]

"So what was in the Constitution in 1787 to stop a state from denying fundamental rights to a citizen of another state, if not Article IV?"

By "denying fundamental rights" of course you mean, "denying the protection of fundamental rights". In 1787, there was nothing to stop a state from denying the protection of fundamental rights to their OWN citizens (much less citizens of another state) except the state constitution.

[Ken H]

In 1787, there was nothing to stop a state from denying the protection of fundamental rights to their OWN citizens (much less citizens of another state) except the state constitution.

If you exclude fundamental rights from "privileges and immunities", then what required states to protect the fundamental rights of citizens of other states that it recognized for its own citizens?

[El Gato]

In 1787, there was nothing to stop a state from denying the protection of fundamental rights to their OWN citizens (much less citizens of another state) except the state constitution.

Actually there was, although AFAIK, it was never appealed to, and thus like the 3rd amendment is judicial terra incognita. Even laying aside the second paragraph of Article VI, see Article III, section 4.