The people that could RJ in my opinion are these (in order of suspect):

-Virgil Minelli
-Gale Bertram
-Ellis Mars


Ellis Mars could be but doesn't matches the physical description (he is quite soft). So... excluded.

Gale Bertram in the last episode of season 3 after Jane tells him that they will meet Hightower in the mall... right when Jane goes out... Bertram makes a phone call. He phones Timothy Carter to set up the trap to Jane? Or phones to RJ so HE can set up the trap? Being or not being RJ he is totally implied (he tells a part of the Blake poem, the newspaper, the tea on the desk) High social status, power, as chief of CBI knows politicians, has friends in FBI and so on. Now I tell WHY he is NOT RJ.
Let's see the show: 
It's a duel between two masterminds. When you start a game, you introduce the players first right? So the first episode introduced PJ and obviuosly RJ BUT! They did it in a mgnificent way, with a fake RJ case so you are unable to figure out that RJ was already been introduced. It's not Gale Bertram, it's Virgil Minelli.

-Virgil Minelli is RJ. The evil mastermind behind everything. He has everything under control till he has to kill Bosco and his team due to give the case back to PJ and Lisbon's team. He makes himself a wayout, so he can control from outside. It's the moment when it becomes a real duel among PJ and RJ. As a declaration of war.

Hightower is one of his underlings and Craig O'laughling knows it. When he enters the chatau and he is discovered he later shows no fear to die so if is mission was to kill Hightower... WHY he jus't didn't kill her? LaRoche is also another member of the gang. he shoots the man who placed the bomb on Lisbon's body, it's shown he waits without a reason outside the car... as if it was planned. Then. He justifies. LaRoche? Why adding that line? Non sense. It was planned.

Minelli is RJ because is not in an highest rank but not in a lower either. Then the last thing:

-Rebecca, when she saw his murder she is overwhelmed by joy. She could feel that way only for RJ. So RJ is in the CBI. Everyone would have noticed Bertram immediately in the CBI. But Minelli? Or Partridge did the handword on his master's orders? That fits. So Partridge killed Rebecca and with Bertram and LaRoche is the third keyman of RJ in the CBI.

When Lorelei told that "shake hand thing" to PJ referred of a situation before her capture, so before season 4 ending. Because later on she was in jail.
So... enough with the "is Robert Kirkland! Is Bob Kirkland!" 
Because Kirkland is NOT RJ, maybe he is the FBI mole of RJ or... is Volkers friend and Volker is RJ's friend wich makes him obviously another of RJ's friends. 

Well thanks for reading my theory. I hope it's uncorrect so not to ruin the final surprise on finding out who RJ is. : )