Final answer:
The EMT should contact a poison control center or emergency department to seek medical advice.
Step-by-step explanation:
The most appropriate action for an EMT to take when a 4-year-old, 15-kg male has ingested an unknown quantity of acetaminophen (Tylenol), is to immediately contact a poison control center or a local emergency department. The ingestion of an unknown amount of acetaminophen can be potentially harmful, especially in children. It is important to seek medical advice to assess the potential risks and determine the appropriate course of action.