Preheat a large griddle or skillet over medium heat.
Spread one side of bread slices evenly with Thousand Island dressing. On four bread slices, layer one slice Swiss cheese, 2 slices corned beef, 1/4 cup sauerkraut, and a second slice of Swiss cheese. Top with remaining bread slices, dressing-side down. Butter the top of each sandwich.
Place sandwiches, butter-side down on the preheated griddle; butter the top of each sandwich with remaining butter. Grill until both sides are golden brown, about 5 minutes per side. Serve hot.