Preheat a gas grill to high. Brush chicken with 1 Tbsp. olive oil; season with salt and pepper. Grill chicken, covered, turning once, until cooked through, 5 to 8 minutes total, depending on thickness of cutlets.
In a large bowl, whisk together vinegar, curry powder and remaining 3 Tbsp. oil. Season with salt and pepper. Add peaches, tomatoes and shallot; toss to coat. Spoon on top of chicken. Sprinkle with chopped herb, if desired, and serve.