Well, outside of Sanborn and his collaborators, who knows. When the puzzles were first revealed and people started trying to crack them, some of them explored out of the box approaches like the design of the sculpture, odd-shaped letters, shadows of the symbols, it's geographic position, etc. However eventually all first 3 turned out to be classic cryptographic algorithms (Vigenere for K1 and K2, transposition for K3), with the information to solve them contained within the cyphertext of the sculpture. For K4, Sanborn has hinted that this may not be the case.