Spread about 1 teaspoon mayonnaise mixture over 1 side of each of 4 bread slices. Top each slice with 1 turkey slice, 1 ham slice, 1 cheese slice, and 2 tomato slices. Top with remaining bread slices.
Heat a large nonstick skillet over medium heat. Coat pan with cooking spray.
Add sandwiches to pan; cook 4 minutes or until lightly browned. Turn sandwiches over; cook 2 minutes or until cheese melts.