What should you do when you're traveling upstream on a river at dusk and you see a buoy with the number four in a flashing red light?

If you see a buoy with the number four and a flashing red light while traveling upstream at dusk, you should be cautious and maintain a safe distance. The flashing red light typically indicates a hazard or warning, such as a navigational obstruction or a danger area. Ensure that you are aware of your surroundings, reduce your speed, and be prepared to alter your course if necessary. Always refer to your navigation charts or local regulations to understand the specific meaning of the buoy.