I think Red John is Harker Rosalind.

In Season 4 Episode  2 Jane takes Rosalind to the morgue to identify Timothy Carter body.  Watch Rosalind and her body language, she seems as if she has seen a ghost and has the look on her face that the man on the morgue table is someone she loves.  Her body language suggests to me that she is deeply connected to the dead man (lover, brother, etc).

Timothy Carter and Harker Rosalind were extremely close together and loved each other.  Carter knew that Jane would not stop searching until Jane kills Red John.  Out of love for Rosalind, Carter pretends to be Red John and knows all Jane's secrets from Rosalind telling him, he fakes being Red John and Jane kills him.  Rosalind now has an opening to stop being Red John and for Jane to stop trying to kill her since Jane thinks he has killed Red John.  After she closes her mouth with the look of horor on her face, she says I don't know this person.  You can see the emotion change at the end of this episode where Rosalind's face goes from grief to anger and says, "I have never met this man." 

In Season 4 Episode 13 "Red is the New Black," Rosalind says Red John is at her home.  Rosalind kills the morgue attendant in anger and to show Jane.  If she has hightened senses from being blind she should know a dead body is in her closet and also show similar emotion from when she was so upset the day she itenditifed Timothy Carter's body in the morgue.  But she doesn't show emotion.

Lastly she has "red" hair. Jane always has is guard down around Rosalind  because he thinks she is blind and innocent.