Which of the following drugs is an example of a Drug Schedule 4 substance?

A drug classified as a Schedule 4 substance is characterized by having a lower potential for abuse compared to drugs in higher schedules, and it is generally accepted for medical use in treatment. Diazepam, commonly known by the brand name Valium, falls into this category. It is used primarily to treat anxiety, muscle spasms, and seizures, making it an important medication in various clinical settings.

In contrast, heroin is a Schedule 1 substance due to its high potential for abuse and lack of accepted medical use. Codeine, although it can be used medically, is typically classified under Schedule 2 or 3, depending on its formulation and concentration. Marijuana's classification can vary by state, but federally, it is considered a Schedule 1 substance as well, reflecting its high potential for abuse and lack of accepted medical use under federal law. Thus, Diazepam is the correct choice as it represents a drug with recognized therapeutic applications while having a comparatively lower abuse potential.