How many episodes  of 30 minutes were written by John Sullivan?
CREATE TABLE "table1_17641206_4" (
  "episode" real,
  "original_title" text,
  "directed_by" text,
  "written_by" text,
  "original_airdate" text,
  "duration" text,
  "viewership" text
);
SELECT COUNT("duration") FROM "table1_17641206_4" WHERE "written_by"='John Sullivan';