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Troponin and its isomers (C, I, and T) are proteins released when the heart muscle is damaged, making them highly specific markers for myocardial injury. In the context of a myocardial infarction, especially four days after an episode of severe chest pain, troponin levels would still typically be elevated, as they can remain detectable in the bloodstream for up to two weeks after the event. This prolonged duration makes troponins the most reliable indicators for diagnosing myocardial infarction, particularly in patients presenting with chest pain that occurred several days prior.

In contrast, while an ECG can provide valuable information regarding the presence of ischemic changes, it is often more useful for early detection of myocardial infarction rather than confirming it days after the event. Similarly, creatine-kinase and isoenzyme (CK-MB) levels start to rise within 4 to 6 hours of infarction but typically return to baseline levels within 72 hours, which limits their usefulness in diagnosing an event that occurred four days earlier. Myoglobin, although it rises early in myocardial injury, lacks specificity and is also not typically elevated by the four-day mark following an infarction.

Thus, troponin testing provides the most accurate diagnostic information in this scenario,