In a small bowl, stir together the mayonnaise, deli mustard, and horseradish if using.
Spread the mustard mixture on one side of each slice of bread.
Divide the corned beef evenly over 4 slices of bread.
Sprinkle the corned beef lightly with black pepper.
Top each sandwich with 1 slice of Swiss cheese.
Add a small amount of well-drained sauerkraut and 2 pickle slices to each sandwich.
Close the sandwiches with the remaining bread slices.
Lightly spread the outside of each sandwich with softened butter.
Heat a large skillet or griddle over medium heat.
Cook the sandwiches for 3 to 4 minutes per side until the bread is golden and the cheese is melted.
Let the sandwiches rest for 1 minute, then slice and serve warm.