In a small bowl combine; mayonnaise, ketchup, sweet pickle relish, and prepared horseradish. Spread on one side of 4 slices of bread.
On the other 4 slices of bread spread on the mustard. Top with Swiss cheese, corned beef, and sauerkraut. Top with second slice of bread that has dressing spread. Butter the outside of the sandwiches.
In a large skillet, toast sandwiches over medium heat 3-4 minutes per side or until golden brown and the cheese is melted.