How many production codes are there for episode number 45?
CREATE TABLE "table1_2602958_4" (
  "no" real,
  "num" real,
  "title" text,
  "director" text,
  "writer_s" text,
  "original_air_date" text,
  "prod_code" real,
  "u_s_viewers_million" text
);
SELECT COUNT("prod_code") FROM "table1_2602958_4" WHERE "no"=45;