1. Spread one side of each slice of bread with butter. Lay ham on unbuttered side of 4 of the slices. Top each with 2 slices of cheese and pineapple chunks. Spread unbuttered sides of remaining bread slices with mustard, if desired. Place those 4 slices, buttered sides up, on top of pineapple.
2. Place sandwiches in a large nonstick skillet over medium-low heat and cook, pressing down on them occasionally with a spatula, until golden on one side, 3 to 4 minutes. Carefully turn and continue to cook until golden on the other side, about 3 minutes longer. Cut each sandwich in half and serve immediately.