ORA-22835: Buffer too small for CLOB to CHAR or BLOB to RAW conversion (actual: …, maximum: …)

create table tq84_t (c clob);

insert into tq84_t values (lpad('x', 4000, 'x'));
select cast(c as varchar2(4)) c4 from tq84_t;
--
-- C4  
-- ----
-- xxxx
   
update tq84_t set c = c || '!';
select cast(c as varchar2(4)) c4 from tq84_t;
--
-- ORA-22835: Buffer too small for CLOB to CHAR or BLOB to RAW conversion (actual: 4001, maximum: 4000)