The fact that you were called back for a fourth polygraph session (rather than being summarily disqualified after an earlier polygraph "failure") suggests that the agency is seriously interested in hiring you. Unfortunately, they place undue reliance on the pseudoscience of polygraphy. It is not uncommon for honest, qualified persons to be wrongly disqualified from employment based on erroneous polygraph results.

I think you might want to re-consider your willingness to work for an employer that has the effrontery to pretend to measure your honesty and integrity based on such a fraudulent procedure as lie detector "testing." If the agency in question is the CIA or NSA, you can look forward to being subjected to the same demeaning treatment every five years (or more, depending on assignments).