Spread about 1 teaspoon mustard over each bread slice. Place 1 cheese slice on each of 4 bread slices. Divide ham evenly over cheese. Cover with the remaining 4 bread slices, mustard sides down. Combine milk and egg whites in a shallow dish. Dip both sides of each sandwich into milk mixture.
Heat a large nonstick skillet coated with cooking spray over medium heat. Cook sandwiches for 3 minutes on each side or until lightly browned. Sprinkle each sandwich with 1/2 teaspoon sugar; top each with 1 tablespoon jam.