Preheat a grill to high. In a dish, combine 4 tablespoons sugar, 1/2 teaspoon salt and 1/4 teaspoon cayenne pepper. Cut 1 honeydew melon in half lengthwise and remove the seeds. Cut four 1-inch-thick C-shaped slices from each half. Thread each slice onto a 12-inch skewer, passing through both ends of the C. Coat one side of each slice with the sugar mixture. Grill coated side down on a well-oiled grate until grill marks appear, about 2 minutes. Sprinkle the uncooked sides with the sugar mixture, flip and grill until caramelized, about 3 minutes.