"Have the contents of the fourth contaminated letter ever been released? Was it a photocopy of the previous letters or something different?"
Fourth? to whom presumably sent?
The others being sent to Tom Brokaw, Tom Daschle, and the editor of The New York Post.
Just one of those nagging questions. That fourth letter gets overlooked.
There are at least 2 different letters (a third was photocopied from one of these two).
The suppression of the content of that fourth letter shows that as much as we do know about this attack, we still have not been provided with at least some of the clues.