The successor for the Massachusetts 20th district was seated on what date?
CREATE TABLE "table1_225098_4" (
  "district" text,
  "vacator" text,
  "reason_for_change" text,
  "successor" text,
  "date_successor_seated" text
);
SELECT "date_successor_seated" FROM "table1_225098_4" WHERE "district"='Massachusetts 20th';