SQL & PL/SQL

I have created a table as below.

create table MYCLOB ( CLOB_DATA CLOB );

Then inserted the below record into it.

INSERT INTO MYCLOB VALUES (RPAD('AAAAAAAAAAAAAAAAAA',40000,'A'));

Then when i try to select the length of the CLOB_DATA column, it shows only 4000.

SQL>  select length(clob_data) from MYCLOB;

LENGTH(CLOB_DATA)

-----------------

             4000

Here why does this insert statement not stored the value with length of 40000.

My Main Requirement is i need to use this logic in reading a Feed File, which will have one of the column with Really Large data. I need to insert it into a table with CLOB datatype column.

Please help.