I've had my Dremel 4000 for over a year, and I've never had trouble with it. Recently though, I've noticed that the tool...

The Dremel 4000's cushioned grip which can make it a challenge to pry apart the casing. The key is to first remove the housing cap which will expose the hard plastic threads of the exoskeleton, then use your prying tool to start a wedge there. This troubleshooting page may have additional useful information.