I believe that Red John is Robert Kirkland. Why? Well first of all he matches Rosalind Harkers description, he also is very well connected in Homeland Security and can go places without raising suspicion. He also killed Jason Lennon and asked him several times if Jason recognized him. This leads me to suspect that Kirkland is not his real name and that it is just one of his fake identities. He probably used another follower to act like Red John with Jason and just wanted confirmation that Jason did not know the "real" Red John. He also is not listed on the season finale cast list, which leads me to suspect that he will appear but that it is supposed to be a secret just like with the fake Red John in the season 3 finale.
