When 4 clubs are involved, what is the average number of fixtures?
CREATE TABLE "format_and_schedule" (
  "round" text,
  "date_s" text,
  "clubs_from_the_previous_round" text,
  "clubs_involved" real,
  "fixtures" real
);
SELECT AVG("fixtures") FROM "format_and_schedule" WHERE "clubs_involved"=4;