What was the crowd size on 4 july 1981, and a Away team of essendon?
CREATE TABLE "round_15" (
  "home_team" text,
  "home_team_score" text,
  "away_team" text,
  "away_team_score" text,
  "venue" text,
  "crowd" real,
  "date" text
);
SELECT "crowd" FROM "round_15" WHERE "date"='4 july 1981' AND "away_team"='essendon';