It's Bob Kirkland.
Not because he's acting strange and makes strang faces all the time.
Remember when he murders the guy in the hospital? He asks him to take a good look at him and tells him if he knows who he is.
Now remember what the false RJ said to Jane in the last episode of season 3 before he kills him? He says it's time he changes his face and moves on!
So Kirkland could be Red John after plastic surgery. So he had an other face, which we used to see in previous seasons. Who could he be? One we used to see, but not any more.