How many years was ensign n180b a Chassis with 4 points?
CREATE TABLE "complete_formula_one_world_championship_" (
  "year" real,
  "entrant" text,
  "chassis" text,
  "engine" text,
  "points" real
);
SELECT SUM("year") FROM "complete_formula_one_world_championship_" WHERE "points"=4 AND "chassis"='ensign n180b';