You need to copy the book file from My Kindle Contents in your Documents folder in Windows Explorer.
Make sure the book has actually been downloaded by Kindle 4 PC! (You will be able to read it in the program.) It will also show you files in the cloud, which doesn't help you find your book files.
Google "apprentice alf" and you may learn something about reading the file too.

Most ebooks come with DRM encryption. But we cannot discuss the details here.