Gently combine first 4 ingredients in a large bowl. Shape mixture into 4 (4-inch) patties.
Grill, covered with grill lid, over medium heat (300° to 350°) 4 to 6 minutes on each side or until beef is no longer pink in center, topping each burger with 1 cheese slice during last minute of cooking, if desired. Grill buns, cut-side down, 30 seconds. Remove from grill, and place burgers on bottom halves of buns; top with remaining bun halves. Serve with desired toppings.