Attached is a link to some of my posts regarding the polygraph with a similar story to yours.
The fact that you have had three polygraphs already with the possibility of taking a fourth one proves that the government is still considering you for employment. If the government did not want to continue, you have received notification that your application is being discontinued.
What is confusing is you being told that you need to reschedule your polygraph, but you may not get another session. I would reach out to your recruiter to get clarification because that is conflicting instructions.