What year made has a Quantity made of 4?
CREATE TABLE "mallet_and_simple_articulated_locomotive" (
  "class" text,
  "wheel_arrangement" text,
  "fleet_number_s" text,
  "manufacturer" text,
  "serial_numbers" text,
  "year_made" text,
  "quantity_made" text,
  "quantity_preserved" text
);
SELECT "year_made" FROM "mallet_and_simple_articulated_locomotive" WHERE "quantity_made"='4';