Then he found a post from 2004 on a defunct Tokyo server. The user, Type_Ghost , had uploaded a "Verified CID Master."

Most "CIDFont" errors occur because the document uses CJK characters and your PDF reader doesn't have the necessary language resources. Go to the official Adobe website.

The "F4" part is just a random identifier assigned during the PDF's creation.

Are you seeing this error in a (like AutoCAD or Word) or just when opening a PDF file ?