I am inclined to believe that the 4th piece is written in a foreign language or otherwise multiply-encoded, or that Sanborn made a drastic error in the cryptography. The other 3 sections were solved independently in a much shorter time. For instance, the NSA was the first to crack the first three parts of the sculpture, and they did it in only 2 days. The fact that nobody has made any progress on the 4th section in well over a decade even with a known clue is hard to reconcile.
> Then “within two days of receiving the information tasking from Chief, Z,” they had solved parts one through three of the puzzle. They spent another day on the fourth section, but very quickly “a decision was made to stop any further work” on it. “Given the suspected cryptography, the last section is too short to solve without diverting a great deal of effort from operational problems,” they wrote in the memo.
Elonka Dunin is the maintainer of the well-known Yahoo Kryptos group, and maintains the primary reference page.
Wired ran an interesting article about the NSA's original crack of the scultpure, which I have quoted from above.
http://www.wired.com/2013/07/nsa-cracked-kryptos-before-cia/