Who was performer 4 when Kate Robbins was performer 3?
CREATE TABLE "radio_series" (
  "date" text,
  "episode" real,
  "performer_1" text,
  "performer_2" text,
  "performer_3" text,
  "performer_4" text
);
SELECT "performer_4" FROM "radio_series" WHERE "performer_3"='Kate Robbins';