Do you see the full 4gb space total or only 0.5gb?
you can do recovery for flash media using the below software
its free and I used it to recover images my brother deleted from the camera..it recovers media files mostly though which seems to be what you want to recover)
http://www.pcinspector.de/SmartRecov...htm?language=1
Note: when recovering make sure to put it on a hd disk and not back on the flash media