Spread mustard over 4 slices of bread. Layer with cheese, corned beef and sauerkraut. In a small bowl, mix mayonnaise, ketchup, relish and horseradish; spread over remaining bread. Place over sauerkraut. Spread outsides of sandwiches with butter.
In a large cast-iron or other heavy skillet, toast sandwiches over medium heat until golden brown and cheese is melted, 3-4 minutes on each side.