[pjbk]

Probably correct. Different from the other cyphers, the number of symbols is short, and correlating part of the plaintext that has been revealed gives poor measures for the full string length. It has been said that the other solutions are required to solve K4, so if the solution relies on something like character alignment, matrix coding or an even more convoluted permutation arrangement, this can look (or directly be) a one-time pad cypher which are arguably the most difficult to solve.

[cyberge99]

Does it’s S shape, or the shadow it casts or any other physical representation of it have to do with the message?

[pjbk]

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.

[willvarfar]

is it hinting at S-Box?