Spread each slice of bread with butter. Spread 2 of the buttered slices with horseradish mixture. Top with roast beef. Top with remaining bread slices, buttered-side down. Trim the crusts from bread. Cut each sandwich diagonally into 2 triangles. (Sandwiches can be filled and refrigerated, covered tightly with plastic wrap, for one hour before serving.) Top each with radishes before serving. Makes 4 bite-size sandwiches.