Sanborn has either honestly suggested there is an over-arching meta-message to the whole of Kryptos or is a talented self-promoter and is encouraging further mystery in the event that K4 is solved and we are all disappointed.
Your suggestion is entirely possible but it seems like it would almost depend on the cleartext of K4 to suggest further searching via cryptology or if the extra message is more philosophical. For now, K4 is hard enough for me without trying to extract more meaning out of the first 3 parts (this doesn’t preclude a different message in the Morse code though).