How to insert or update more than 4000 byte in Oracle 11g?

Question

Hope someone help me how to insert or update more than 4000 byes in CLOB. I used PL/SQL and only gets 01704. 00000 - "string literal too long"

CREATE TABLE don (x clob);

DECLARE 
 l_clob clob;
BEGIN
  FOR i IN 1..10
  LOOP
    INSERT INTO don (x) VALUES (empty_clob()) --Insert an "empty clob" (not insert null)
    RETURNING x INTO l_clob;

    FOR i IN 1..100
    LOOP
      dbms_lob.append(l_clob, rpad ('*',4000,'*'));
    END LOOP;
  END LOOP;
END;
/
Insert into don (x) values ( to_clob( 'String value more than 4000' ) || to_clob( 'value of remaining 1500 characters' ));
insert into don (x) values('String value more than 4000');

can anyone find out which thing I got wrong..?