In a small bowl, COMBINE mayonnaise, ketchup, relish and horseradish; SET aside.
SPREAD mustard on one side of four slices of bread; layer with the corned beef, cheese, sauerkraut and mayonnaise mixture; TOP with remaining bread slices.
In a large skillet, MELT butter over medium heat.
ADD sandwiches; COVER; COOK on both sides until bread is lightly toasted and cheese is melted.
Per Serving (excluding unknown items): 1099 Calories; 67g Fat (54.1% calories from fat); 67g Protein; 61g Carbohydrate; 16g Dietary Fiber; 149mg Cholesterol; 5335mg Sodium. Exchanges: 2 Grain(Starch); 7 1/2 Lean Meat; 4 Vegetable; 7 Fat; 1/2 Other Carbohydrates.