"Now it came about when Pharaoh had let the people go, that God did not lead them by the way of the land of the Philistines, even though it was near; for God said, 'Lest the people change their minds when they see war, and they return to Egypt,'" (Exodus 13:17).
This verse, like Exodus 4:9, is a conditional statement where God knows the future and states what will happen if different choices are made, i.e., "If you do this, then this will happen; if you do that, that will happen." Since the Lord is all knowing, He knows what will happen if certain conditions occur. In Exodus 13:17, God was stating that He knew what would happen to the people's faith if they were to face the Philistines and war. So, He led them in a different direction. This demonstrates that God knows the future. He knows what will happen under different circumstances.
If, as Open Theism states, God does not exhaustively know the future, but only the present, then how would he know what the Jews would actually do when facing the Philistines? At best, God could only guess. Is this what we have here? Is God guessing? Is He hoping for the best by trying to avoid war with the Philistines so that His people wouldn't loose faith? Is this the confidence building idea that the Open Theists have for us?
Instead, God is simply telling us what would happen under different circumstances, demonstrating that God is, indeed, all knowing (1 John 3:20).