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The number 4 in the hazard diagram, particularly in the context of the National Fire Protection Association (NFPA) label system, indicates a high level of reactivity hazard. This means that the material can easily undergo hazardous reactions under normal conditions, which can lead to explosive incidents or the release of harmful gases. The scale ranges from 0 to 4, with 4 representing the most significant danger, thereby requiring strict safety measures and precautions when handling or working in proximity to the material designated with this rating.

The distinction between hazard types is crucial. For example, a flammability rating would denote how easily and intensely a substance can catch fire, while a health hazard rating would focus on the harmful effects of exposure, such as toxicity or disease potential. Similarly, radiation hazards specifically relate to the dangers posed by radioactive materials, which are measured using a different set of criteria. Understanding these different categories and their implication on safety practices is essential for anyone working in or supervising construction environments where hazardous materials are present.