Comment by Prozac on the YAR in Kryptos.

“Here is my opinion of the YAR in K3. I actually have 2 theories. Here’s the first. If you use the Vigenere method with the same pass keys as K2 (Kryptos and Abscissa) but instead of decrypting it you actually encrypt it. You end up with CIA. Coincidence? Maybe. It could be a clue that tells us that we need ENCRYPT K4 not decrypt it in order to get a readable message.”

Sounds like they have an interesting and different take on K4.