Fold 4 (16 x 12-inch) sheets of heavy-duty foil in half lengthwise. Open foil, and layer 4 potato pieces, 1 onion slice, 1 bell pepper ring, 4 tomato pieces, 1/4 cup carrot, and 1 tablespoon margarine on half of each foil sheet; sprinkle with salt and pepper. Fold foil over vegetables; tightly seal the edges.
Prepare grill. Place packets on grill rack, and grill 15 minutes; turn packets, and cook an additional 10 minutes or until potato pieces are tender.