CREATE TABLE table_2160008_4 (reason_for_change VARCHAR, date_successor_seated VARCHAR)

Name the number of reason for change on may 11, 1939
SELECT COUNT(reason_for_change) FROM table_2160008_4 WHERE date_successor_seated = "May 11, 1939"
SELECT COUNT("reason_for_change") FROM "table_2160008_4" WHERE "May 11, 1939" = "date_successor_seated"