Why did God keep the flies out of Goshen during the fourth plague?
At the fourth plague, God placed a division between Egypt and Goshen to prove His power and His special dealing with His covenant people. Unlike the first three plagues, which affected all alike, this distinction showed that Israel was set apart, so no flies came to Goshen where they dwelt.
Les explains that God had set Israel apart from the beginning, calling out Abraham, Isaac, and Jacob and letting them know they were to be a sanctified, set-apart nation. When the Exodus plagues came, the first three struck everyone alike, with no division. But at the fourth plague, the flies, God told Pharaoh He would put a division between His people and Pharaoh's people. He did this to prove His power and to prove His dealing with His covenant people, making a separation between Egypt and Goshen, so there would be no flies in Goshen where Israel dwelt.