I am nearly 100% confident I know Brett Partridge is Red John. A while ago I was on a behind the scenes tour at universal studios where most of The Mentalist is filmed. I was lucky enough to meet some of the cast, Jack Plotnick (Brett Partridge). They mentioned that they were doing some filming for season four. Now, Brett Partridge does not appear as himself in season four, but Red John is seen holding the tea cup. The only logical conclusion is that Brett Partridge is Red John: