Name the production code for 4.92 million viewers
CREATE TABLE "table1_23117208_4" (
  "no_in_series" real,
  "no_in_season" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "prod_code" text,
  "viewers_millions" text
);
SELECT "prod_code" FROM "table1_23117208_4" WHERE "viewers_millions"='4.92';