In a small bowl, stir together cream cheese and mustard. Spread evenly on 4 slices of bread. Sprinkle each evenly with cheddar cheese and top with remaining bread. Lightly coat outside of bread with cooking spray. Heat a large nonstick skillet over medium heat. Place 2 sandwhiches in skillet and cook 3 minutes on each side. Repeat with remaining sandwiches. Serve immediately.