Only if James Sanborn is the lamest, most unimaginative person on earth.
In other words, no, K4 is not a Vigenere cipher because K1 and K2 are Vigenere ciphers. I hope efforts have been made to check but I wouldn’t blame anyone for not wasting their time. Could it be? Oh yeah! Definitely. Absolutely. Positively. What are the chances that it is? Really, really low. Low enough that I would recommend to anyone that they not waste their time on it.
You will get folks who want to use it in combination with something else in K4. This is possible. A Vigenere would alter the letter frequencies and if you used it in combination with a transposition then it would indeed provide a challenge to decryption efforts.
How incredibly disappointing if that’s how it was done though. It would mean that Sanborn basically used 2 cipher methods (Vigenere substitution and transposition) for what is widely touted as one of the most famous unsolved ciphers of all time.
Plus, if Vigeneres are vulnerable, would you really want that last bit which was supposed to be the real challenge to involve just a trickier use of the same old, same old?
I have no well-founded logical arguments to present here. If you believe the interviews then you have enough evidence that one could make an argument that it’s not a Vigenere.
It will boil down to making an opinion.
Do you think Sanborn would use the same cipher for 3 out of the 4 parts of his monument to cryptology?
Would he use a cipher that was ironically known as “unbreakable” (le chiffre indéchiffrable).
How likely is that?
What are the odds of that?
Is that something you would do?
Is that something Ed Scheidt would let him do?
Would it be possible for it to resist cryptanalysis for 20 years?
If it really is, do we want to know and forever have our memories of Kryptos tainted?
Personally, I think it is so very unlikely that I won’t even consider it. I also have higher standards of a riddle/cipher sitting in the courtyard of the CIA and would rather waste my time on better cryptosystems then find out it was just one more Vigenere.
I can’t say that it’s impossible and besides the interviews I have no evidence at this time to make a conclusive argument against a third Vigenere in play. I just have a really, really strong opinion and a feeling.
I might be wrong.
I hope not.