Name the number of reason for change on may 11, 1939
CREATE TABLE "table1_2160008_4" (
  "district" text,
  "vacator" text,
  "reason_for_change" text,
  "successor" text,
  "date_successor_seated" text
);
SELECT COUNT("reason_for_change") FROM "table1_2160008_4" WHERE "date_successor_seated"='May 11, 1939';