In a small bowl whisk together oil, vinegar, salt, and pepper. Brush 1 tablespoon of the oil mixture over the tenderloin. Reserve remaining oil mixture to use in the relish.
For a charcoal grill, arrange hot coals around a drip pan. Test for medium-high heat above pan. Place meat on grill rack over pan. Cover; grill for 30 to 35 minutes or until a meat thermometer registers 155 degrees F. Remove meat from grill. Cover with foil and let stand 10 minutes. (The meats temperature will rise 5 degrees F during standing.)
Meanwhile, for relish, cut either the cantaloupe or honeydew melon half into bite-size pieces. Cut remaining melon half into four wedges. In a medium bowl combine melon pieces, tomato, onion, and mint. Add reserved oil mixture; toss to coat.
Slice pork diagonally. Serve with relish spooned over melon wedges. Makes 4 servings.