I don't think MII supports true CLOB and BLOB data types so you are hitting the size limitation of the data type its actually using. 4000 characters I think. Google 'sap xmii clob' or similar and you should be able to find a lot a threads discussing this. I know others have sent xml via queries in the past so if you dig around you may be able to find a solution unless somebody chimes in. I have been able to read clob data by using functions to pull it out in chunks. You may be able to insert it in chunks. I think this issue is typically solved with a custom action.
Regards,
Christian