Ask any Roman Catholic and the earliest possible time of a church father claiming Mary was assumed into heaven, and the earliest they will come up with is the 4th century. Since the Bible was already completed in the 1st century what does it matter what some guy says in the 4th century?

It was already determined that Mary was not assumed since there is no such indication in the Scriptures, that is, the 66 books of the Bible. And there are verses that say nobody is in heaven yet, not even David a man after God's own heart (Acts 2.34).