@remington There are few creators whose videos I will jump to view the instant they drop, and Lemmino is one of them. This is a pretty interesting subject that I haven't heard of, despite it apparently being quite well-known.
Tbh, Sanborn not being confident/experienced with math and cryptography kinda tracks with his apparent surprise that expert cryptographers cracked a Vigenere cipher in a couple days rather than follow an obscure breadcrumb trail that's still unclear, even after knowing the key. For me, K4's enduring mystery prompts comparison to the Zodiac killer ciphers, which ended up being so difficult to unwind not because they were brilliant ciphers devised by a mastermind, but because the author made a bunch of mistakes. Still, at this point it seems likely that Sanborn has checked his work over multiple times, so maybe there really is just some trick that no one has thought of. He's clearly eager for it to be solved, so we may know in the coming decades!