If you remember the interview yesterday, he directly said. "I started with a back 4 line, intending to change it after some minutes, but we all saw what happened.."
An in fact, if you watch carefully, yesterday Busquet played practically as a CB, opening up Puyol to the right, and pushig Dani as a RMF... In fact, at the second period Barça started the "infamous" back 3 line promised, but app for 45-60 minutes Barça played a back 4...
I think Barça will always look to use a back for, with a more defensive left back, thus the before mentioned "zona mista", obviously modified to a MUCH more offensive strategy. But if you carefully compare a full attacking side, like, for example, Bielsa's Athletic, or Universidad de Chile in South America, you can clearly see that Barça isn't an attack animal, but a posession beast.
Even Guardiola has mentioned many times that his teams flow during a match, so therefore, we need to create a PES tactic that reflects this movement, and after trying it out a huge deal this past month, personally an irregular back 4 has proven much more efficient than a back 3.
In a few days I'll post my formation with Xyder, don't worry