Combine ground beef, water chestnuts, 4 tablespoons of the green onions, soy sauce, garlic powder and pepper in medium bowl. Shape mixture into four burger patties about 1/2-inch thick. Set aside.
Spray grill pan or grill with grilling spray. Heat over medium-high heat. When hot, add burger patties and cook 5 to 7 minutes per side or until juices appear on surface and burgers are browned. Turn and cook remaining side until no longer pink in center.
Meanwhile, combine mayonnaise, sweet and sour sauce and the remaining 2 tablespoons green onions.
Spread 1 tablespoon sweet and sour mixture on EACH half of buns. Add burgers plus lettuce, tomato and/or pineapple, if desired. Serve immediately.