Combine instant potatoes, horseradish and 1 Tbsp.of melted butter. Stir in the milk until well combined. Rinse the fish filets and pat them dry. Combine flour with salt and pepper and dredge the fish in mixture. Divide the horseradish/potato mixture into four equal parts and spread on the top of the floured fish filets. Sprinkle the top of the prepared fish with bread crumbs and drizzle the remaining 3 Tbsp. of melted butter, evenly over all of the fish. Place on a cookie sheet and bake for about 15 minutes at 375 degrees.