What specific action must be taken when a 4-inch flat spot is discovered on a wheel?

When a 4-inch flat spot is discovered on a wheel, the train must be immediately stopped. This action is crucial because a flat spot can lead to significant safety hazards, including potential derailments or damage to the rail infrastructure. A flat spot on a wheel impacts its ability to make proper contact with the rail, which can cause vibrations, instability, and further wear on the wheel and the track. Stopping the train ensures the safety of both the passengers and the crew, allowing for an assessment of the situation and prompt action to address the issue.

The other choices do not prioritize safety and could lead to severe consequences. Skipping the notification process or moving the train at high speed disregards the immediate dangers posed by the flat spot, while notifying only the conductor would not adequately address the problem or mobilize the necessary responses to ensure the train's safety and integrity.