I'm not sure the question is clear. You state that you can't save the MP4's once they are downloaded? By default, they will likely download to your "Downloads" folder on your local machine (if on Windows).
If you mean the "Download" button is not available, this may be a setting controlled by your Webex administrator. There are admin options available to prevent the downloading of recordings.
Your Webex admin should also have access to download your recordings on your behalf. Perhaps you simply need to ask them to archive them somewhere for you before disabling your account.