So if a PRNG was utilized, what is a cryptosystem that would employ it?

Stream Cipher

There is a benefit to considering stream ciphers as they are fundamental to much of modern cryptography.  With only 97 letters and without multiple samples of ciphertext and the hope that the same key was used… from what I’ve read so far, these seem to be secure ciphers especially in the c0ntext of K4.

Why then would I or anyone else bring it up?  Well, because there may be someone out there who has the knowledge and ability to check something like this so it’s useful to bring it up.  Also, there are always primitive ancestors and it’s not impossible that knowledge of simplified types of stream ciphering was known by Ed Scheidt and used in Kryptos’ fourth part.  This implies analysis is needed by someone familiar with these ciphers to see if there’s any direction given in the text of Kryptos that would enable one to break a stream cipher of K4.

That’s not going to be me.

The type of ciphering is appealing for K4 but stream ciphers are an example of symmetric key ciphering.  This means that sender and recipient must possess the seed/key as well as the ciphering-PRNG algorithm.  These will need to be recovered if there is to be a hope of success.  So if you’re interested in antecedent-stream ciphering I wish you the best of luck but I won’t be able to help you at this time.