Heat a grill to high (450° to 550°). In a small bowl, combine hot sauce, brown sugar, vinegar, and ketchup. Put half the mixture in a shallow bowl and reserve the other half to serve as ketchup. Season steak with salt and pepper. Add steak to bowl and turn to coat; marinate 10 minutes.
Grill steak, brushing occasionally with any remaining marinade as you go, 5 minutes per side for medium-rare. Remove from heat and let rest 5 minutes.
Serve with reserved ketchup and, if you like, fries.