there are some possible reasons for this problem.
first of all - the 300d normally does not know 4gb cf cards
did you update to the latest firmware ?
there are ways to format a cf card different - so the camera works better with them
but the eos had sometimes problems with the 4gb: [http://photo.net/canon-eos-digital-camera-forum/00MoAM|CLICK]
try to update the camera and format the card like mentioned in the link above