I'm not entirely clear if you want to change the video format or just the container format. If you just want to write the existing camera output into a different container file, then you need a multiplexor filter. There's an MP4 multiplexor filter available in source form at www.gdcl.co.uk/mpeg4. If you connect your camera's output to this filter and then the file writer, you should be ok.
If, on the other hand, you need to encode the camera output to mpeg-4 video as well, then I think you will need to licence a third-party encoder filter.
G