CREATE OR REPLACE PROCEDURE prc_test (par_out OUT VARCHAR2)
IS
  l_out VARCHAR2(4000) := 'x';
BEGIN
  FOR i IN 1 .. 3200 LOOP
    l_out := l_out || 'x';
  END LOOP;
  
  par_out := l_out;
END;
/

DECLARE
  v_out VARCHAR2(4000);
BEGIN
  prc_test (v_out);
  dbms_output.put_line('Length = ' || TO_CHAR(LENGTH(v_out)));
END;
/

Length = 3201