When william b. campbell (u) is the successor what is the district?
CREATE TABLE "table1_2147588_4" (
  "district" text,
  "vacator" text,
  "reason_for_change" text,
  "successor" text,
  "date_successor_seated" text
);
SELECT "district" FROM "table1_2147588_4" WHERE "successor"='William B. Campbell (U)';