I dunno, maybe.
I think Vorlath will figure it out before me but it’s worth considering.
Straddling checkerboards fall under a substitution cipher because they convert plaintext into numbers. These numbers can also be converted back into letters and also modified by keywords.
You build a 4×11 grid with numbers across the top row, the 8 most common letters under those, label the last two rows with the numbers that didn’t get letters and then fill in the rest of the grid:
Each letter can now be replaced by its coordinates. This diffuses the message across a monome-dinome cipher and also suppresses letter frequencies that might otherwise help us.
It’s a good bid for K4.
I can’t attempt it because I don’t know how to set up the encryption/decryption grid for Kryptos, I don’t know if a keyword was applied to the letters and what it would be, and I don’t know whether this is the only ciphering mechanism used on K4 or if it comes before or after the masking technique. I could probably spend a lot of time on it but it would be more of a matter of hoping that I got lucky and hoping someone found a solid method for identifying keywords/clues.
There are some folks who are trying to sort this one out but I don’t have enough solid evidence to make my own attempt. If it’s doable, let’s hope they have some luck.
Again, this would be a very logical method for Sanborn to choose to use so it should be considered.