if you bought them new check the box as there should be adapters in there that slot in the tubes to allow for use of 4mm bullets .... if you dont have the adapters they can be bought from most shops ... or I have seen dms have got some vampire bullet connectors that are duo shaped to allow for use in both 4mm tubes and 5mm tubes.
see here:
http://www.dms-racing.com/index.php?...category_id=55