I just found something that could be a major clue...

Red Sky in the Morning, season 2 finale, Jane and Red John come face to face. Red John enters, shoots the copycat killers, Jane falls to the floor in his chair, Red John cuts the victims open, lifts Jane up and recites the Tyger poem. Then he appears to go away but comes back. Then the interesting part happens.

He strokes Patrick's shoulder twice and then pats it. 

Video to the exact moment: http://www.youtube.com/watch?v=ns_DWnqrEdI#t=110s

Sounds familiar? Remember the episode when Rigsby was hypnotized and the only way to bring him out of his trance was by doing a certain pattern only the hypnotist knew used to make him go into such trance (the trigger)? That's episode Russet Potatoes, from season 1. What was the pattern that snapped Rigsby out of his trance? You guessed it - tapping his shoulder.

Something even crazier happened this season though. Behind the Red Curtain. Robert Kirkland and Jane are at the hospital at the beginning of the episode. Guess what.

Kirkland strokes Patrick twice while saying "Look at you, you're a mess." This video shows the exact moment: http://www.youtube.com/watch?v=D3MRlGFmPjw#t=38s

And then, at the end of the episode, he pats Patrick (although, this time, just once): http://www.youtube.com/watch?v=OfhfOhB7Qxc#t=203s

And finally, something else to point out. Kirkland was drinking a Bloody Mary in episode Red Letter Day: http://www.youtube.com/watch?v=2i5TSyRONTI#t=213s

The same drink Patrick was drinking in Red Rover, Red Rover, for the anniversary of her wife and child's death (in S04E23): http://www.youtube.com/watch?v=5pHcS1aXYeo

As much as I hate to say it, Kirkland may be Red John after all. How else could he have known about the trigger?

I have a theory about that, but maybe it's way too far-fetched. I think it'd explain a few things if Red John was someone who worked at the psychiatric hospital Patrick was in after his wife and child were killed. Perhaps he was hypnotized as a patient and that was the trigger they used for their patients. That could also explain how they got to know about Patrick's happy memory without him even knowing that. Maybe Kirkland found out about the pattern too (we know he's been following Jane closely for years).