Mix cinnamon and sugar together in a small bowl. Spread one tablespoon of peanut butter onto one side of 8 slices of bread.
Arrange apple slices on 4 slices of bread. Sprinkle the cinnamon/sugar mixture evenly over the apples. Top with the remaining 4 slices of bread, peanut butter face down.
Melt the butter in a large skillet over medium heat. Fry sandwiches until browned, about 1 to 2 minutes on each side.