Cover the raisins with hot water and soak for at least 10 minutes (the warm water will help the raisins plump up); drain and place in a large bowl. Add the chicken, celery, onion, carrot, curry powder, and mayonnaise. Mix well and season with salt and pepper. Place the lettuce leaves on top of 4 bread slices, then top with tomatoes, chicken salad, and the remaining bread.