Special Formating in RLDC report Field

Hello Friends;
I want to format a number which shown in the report at runtime.
I have a number 1,2,3 but i want to show as 0001, 0011 and so on.
i use in expression like
= Format(STR(Fields!ID.Value),"0000")
But it generates ERROR Then i use following
=CInt(Format(STR(Fields!ID.Value),"0000") ) because in the database .mdb file the data type is Long Integer.

But i am not sucessful

Please help me how i fix it
Regards