Final answer:
In the NFPA 704 rating system, the number 4 denotes the highest level of hazard, indicating that a substance poses a severe threat across various categories like flammability, health, reactivity, and special hazards.
Step-by-step explanation:
In the National Fire Protection Agency (NFPA) 704 Hazard Identification system, hazards are rated on a scale from 0 to 4, with 0 indicating no hazard and 4 indicating a level of hazard that is extremely hazardous.
Each hazard such as flammability (red), health (blue), reactivity (yellow), and special hazards (white) are given a number that communicates the degree of risk and necessary precautions associated with the substance. When looking at the NFPA 704 diamond, the number 4 represents the highest level of hazard within this system, which means that it denotes a substance that poses a severe threat.