What is the date made that has an associated fleet number of 4?
CREATE TABLE "londonderry_and_lough_swilly_railway" (
  "type" text,
  "fleet_numbers" text,
  "quantity_made" real,
  "manufacturer" text,
  "date_made" text,
  "date_withdrawn" text
);
SELECT "date_made" FROM "londonderry_and_lough_swilly_railway" WHERE "fleet_numbers"='4';