If the location is Chicago, Illinois, what is the airport name?
CREATE TABLE "table1_18047346_4" (
  "rank" real,
  "airport_name" text,
  "location" text,
  "iata_code" text,
  "passengers" text,
  "pct_chg_2009_10" text
);
SELECT "airport_name" FROM "table1_18047346_4" WHERE "location"='Chicago, Illinois';