"But it shall be that if they will not believe even these two signs or heed what you say, then you shall take some water from the Nile and pour it on the dry ground; and the water which you take from the Nile will become blood on the dry ground," (Exodus 4:9).
This verse does not mean that God doesn't know what will happen. First of all, because God knows all things (1 John 3:20), He can tell us what will happen in the future if different choices are made by people in the present. Therefore, if God says, "If you do this, then this will happen, and if you do that, that will happen," this is not showing uncertainty. On the contrary, it is showing that God absolutely knows what will happen given different options. IF God did not have exhaustive foreknowledge of the future (both actual and potential), then how would it be possible to tell us what would happen?
Second, God works within the time realm of people. This is a fact since we see in the Bible how God tells people what will happen to them later in life. These prophecies do not come true until later--until after time has passed. Therefore, God--in our perspective--waits and accomplishes His desires sometimes after long stretches of time, proving that God works with us in relation to our time frame. In this relationship to time, God will then give us conditional predictions, that is, if someone does one thing, then a result will happen, and if he does another, another result will occur.
This verse does not necessitate that God does not know the future. If anything, it demonstrates that He does.