Name the total number of reason for change for not filled this congress
CREATE TABLE "table1_225200_4" (
  "district" text,
  "vacator" text,
  "reason_for_change" text,
  "successor" text,
  "date_successor_seated" text
);
SELECT COUNT("reason_for_change") FROM "table1_225200_4" WHERE "date_successor_seated"='Not filled this congress';