If you need to revert to StoryMill 3, you can download a copy from our downloads mirror site here:

https://s3.amazonaws.com/MarinerDownloads/index.html

Version 3.2.3 was the most recent release of StoryMill 3.

Once you have downloaded verison 3, just delete your existing copy of StoryMill 4 by moving it from your Applications folder into the Trash, and emptying the Trash.  Then move StoryMill 3 from the DMG you download from the above link and move the application into your Applications folder.

Note that no settings or user data are stored wtihin the application file itself.