In small bowl, combine sauerkraut, cheese, and dressing. Spread each bread slice on one side with butter. Spread half of sauerkraut mixture on unbuttered side of 4 bread slices; top each with 2 slices SPAM. Cover with remaining sauerkraut mixture. Top with remaining bread slices, buttered-side up. In skillet or griddle over medium heat, grill sandwiches until cheese melts and both sides are golden brown.