how many people wrote the episode directed by rob schrab?
CREATE TABLE "table1_28081876_4" (
  "series_no" real,
  "season_no" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" real
);
SELECT COUNT("written_by") FROM "table1_28081876_4" WHERE "directed_by"='Rob Schrab';