Why does prophet Isaiah believe the world is flat?
He will raise a banner for the nations and gather the exiles of Israel; he will assemble the scattered people of Judah from the four corners of the earth. - Isaiah 11:12-13
This is not a literal meaning but the way people speak. We still speak of "sunrise" and "sunset". The idiom of "the four corners", means the farthest away that one can go in any direction.
One example can be seen in Megillas Esther when it refers to מהודו ועד קוש One of the possible explanations is that they were next to each other and he ruled from the far side of one back around to the far side of the other.