Z-scores only matter when you look at more than one sample like we have in this case. You want to not just look at how much better than the average a person did, but we want to compare this for all four exams. To keep them on the same basis, find a mean and standard deviation for each exam. Figure out z-scores for each exam a person takes. Add up their z-scores... highest z-score is the best performer of all the exams collectively.