I believe that Teresa Lisbon is Red John for multiple reasons.
1) If you look at the name Teresa Lisbon, and scramble the letters around, it makes Tesa is red Jon. The J and d are the L and b in Teresa backwards. Teresa is also often spelled with an h, so that would make it Tesa is red John.
2) There is nothing that specifically states the Red John is, in fact, a man. People do say that there are instances where associates and victims of Red John refer to RJ as a he, but do you actually think that Red John would let anyone know who they really are? I'll get to this in a moment.
3) 'He is many'. MANY. The point of the show is that literally ANYBODY could plausibly be Red John. We hear Red John speak to Patrick, but we don't know that's actually the original. Lorelai (I think that's how you spell her name?), Rebecca (secretary who kill Bosco) and Red John's blind girlfriend all believe Red John to be male. But, as I've said, they could just be another disciple that RJ put there to pretend to be RJ. It does seem like the sort of mind-fuckery Red John would pull.
4) Red John doesn't know about Jane's plan until he tells Teresa, which is why it seems RJ is always one step ahead.
5) Those times when it doesn't seem possible that Teresa could kill all those people due to the fact she was with Jane during the murder(s)? Again, it could have easily been a disciple. Plus, we don't know much about Lisbon's personal life. What does she do at night? We don't know, that's thing.
6) Jane's wife and daughter were killed by the original RJ. RJ likes control, he likes to do things himself. If somebody insulted him on national television, he takes care of it himself, he doesn't send another person out to do it. Teresa seems like the type to deal with insults herself as well, doesn't she? Also, while Teresa doesn't normally seem particularly manipulative or controlling, think of how much power she has over Jane. Remember the episode where Jane didn't have an interest in the case so Teresa had to convince him to take it? She was manipulative then.
7) In the season 4 finale, Jane's plan seems to be going well, but it all goes to hell once he tells Teresa. Hmmm, I wonder why? Red John asks for Teresa's head, knowing he would never be able to do it, and also knowing that he wasn't actually going to join him. Plus, who would suspect a person who literally had a price on their head from the murderer? It almost seems too perfect.
8) "All the things Red John does, he does out of love."- Bret Stiles. Maybe they weren't great friends at first, ut Lisbon eventually grew to care for, and even love Patrick. Who has been killed by Red John since the death of Patrick's wife and daughter? Murderers and copycats, mostly. Bosco's team, as well. Patrick's family's death changed him, but what if it changed Red John as well? Comparing him now, almost seeming to care about Patrick, to the beginning, when Patrick was just a toy, another thing to break, it seems a bit like how Lisbon and Jane's relationship has progressed throughout the series. "Red John wants you back."- Rebecca (after she if apprehended for the murder of Bosco's team). Teresa didn't seem too... happy about Bosco's team taking the case, did she?
I could go on, but I expect you are all getting tired of hearing me ramble on, but I am going to make one last point.
9) 'Tiger, Tiger' poem. Jane represents the lamb (obviously), and RJ represents the tiger (duh). Patrick is redemptive, sympathetic, and an all-around good guy (as in, he's the protagonist). He doesn't like guns or violence (normally). He's obsessed with killing RJ, and Lisbon is always trying to talk him out of taking revenge (hint hint). RJ is portrayed as controlling, somebody with demons. Sounds like Lisbon, doesn't it? Of course, there's also instinct that I'm going on here (okay last point, I swear). Ever since the pilot, something about Lisbon always sets off alarm bells in my head. At first, I was in denial- how could she be RJ? But after learning about the powerful force RJ has backing him (or her), I started looking things over. I noticed all of this, and more, in my search.
Besides, Lisbon is a character we truly care for. It seems like the sick sort of thing the writers would do. After all, how can Jane kill someone who's always been there for him, who he cares about and loves? Either Lisbon is Red John, or she's going to die. I believe the former.