I still think it's Grace. Besides what has already been said, isn't it suspicious that nothing really has happened with Red John while the actress has been on maternity leave?

She has red hair.  Mysteriously joined the team as Jane was recruited.  I am British watching it on British telly.  At the moment Van Pelt has gone off on a computer course.  Allegedly had a boyfriend with links to RJ.

I actually think she is not Red John cuz obvi he is a guy. hE IS. But you known how Jane accuses than someone is working in the FBI for RJ? But I also think that Van Pelt is working with RJ and the guy from FBI if there is one. Someone else said this and I agree as well: that she joined shortly after Jane joined CBI and also most associates of RJ wear red (she has red hair and Lorelei had that red jumpsuit )

I say this because everyone she ends up dating is some psycho path. Maybe this is a way they can pass notes about Jane. 
Also she is always staying back at the desk, which could give her enough time to get more rj stuff done.
Also she applied for a job shortly after Patrick Jane became a member of the cbi. This could have been a master idea from RJ.
Lastly her hair is red

Just thought about the timeline at the farm. It was 25 years ago and grace seems to be in her early 30's of at least the actress is about 30. The guy at the farm mentioned a kid and Jane thought he meant 20 -25 but what if the kid was actually 5-6. The two guys could've tried to rape her or her sister so she and someone there killed them. Plus it would fit that RJ had accomplices and helpers early in his/her career which would possibly mean gvp needed it where she was young and weaker. Her sister might have been killed by those two guys or raped then killed herself later in life due to trauma.

I have always suspected Van Pelt since the start. My theories may be flimsy but just a few things I believe, Red hair for a start. In the pilot Van Pelt is introduced to Jane and shakes his hand, she also has a go at him around the dinner table showing a complete hate for him. Shooting her fiance is also intriguing, was he doing it for her? When pregnant in real life amanda righetti was kept in the show when other programmes would have written her out until she was ready to come back, I know she has now finally been written out with the story of her going on a course but it just seems weird? Like I say I've put this on as a little guess for fun but I have always suspected her. I wonder if I will be true. Is red john definately going to be revealed in season 5? I am a viewer in the uk so may be a bit behind you in america.

Van pelt is red John. She is always knows what janes is doing and is always nearby when something odd happens and Jane is getting close