The past three months have formed the bulk of my observations and characterizations of Kryptos and the unsolved K4.
I will now attempt to form a testable hypotheses to explain what I have observed.
A hypothetical explanation of the encryption of K4 is as follows. Working together, James Sanborn and Edward Scheidt developed a plain of ciphering to include the Morse Code outcroppings, the Copperplate and potentially one last piece. The material to be enciphered was at Sanborn’s discretion to some extent with some constraints as far as word/letter number and possibly misspellings or other modifications essential the ciphering systems and their later analysis. K4 allowed 90 letters with no punctuation and was to include the additional word kryptos in the message. Sanborn was to write a message of significant context (#1) with the total 97 letters. Then, the separate 90 letters were enciphered with a digraphic substitution cipher (#2). At this point the seven plaintext letters “kryptos” were added back into the message (#3) and a transposition cipher used to separate the digraphs (#4) which would make cryptanalysis difficult. It is possible that step 4 could have preceded step 3.
For a hypothetical solution, the steps of K4′s encryption are reversed. Since it is difficult to know when “kryptos” was added sequentially, it will be removed first. The remaining 90 letters will be transposed and the transposed letters will then be used in a digraphic deciphering in an attempt to recover the plaintext.