Explanation:

Let's examine the options:

- The original phrase "has not yet known" is incorrect. The passive construction requires a form of "to be" before the past participle.

- Option 1, "No substitution required," doesn't fix the grammatical error.

- Option 2, "have not yet been known," uses "have" for a singular subject ("cause"). It's grammatically incorrect.

- Option 3, "was not yet known," uses past tense, inconsistent with the present tense "has been set up." It suggests the cause was once unknown but isn't anymore.

- Option 4, "is not yet known," uses present tense, aligning with the ongoing situation described. It's grammatically correct.

Therefore, Option 4, "is not yet known," is the correct substitution. ""