When semi final (2nd leg) is the round what is the highest attendance?
CREATE TABLE "table1_28181401_4" (
  "date" text,
  "round" text,
  "opponent" text,
  "result" text,
  "venue" text,
  "scorers" text,
  "attendance" real,
  "match_report" text
);
SELECT MAX("attendance") FROM "table1_28181401_4" WHERE "round"='Semi Final (2nd leg)';