I really have no idea how Real Madrid won this El Clasico by 4-0 away. It was a Copa del Rey game maybe but both teams were caring about the cup anyway.
Barcelona actually started better but after some time Real Madrid started to play really amazingly. But I still can't believe Barcelona about how easy they lost their concentration this much. They even had a 1-0 score advantage before the beginning of this game. They were just demolished in their home stadium. Real Madrid should be able to go for the cup as well from now on. They really want this obviously.
Barcelona didn't play in the optimal line-up + Benzema on fire + sooner or later Barcelona's luck had to run out. Yesterday all three factors coincided. Xavi, as always, could not do anything when the game did not go according to plan and in fact, after the second goal, everything was clear, although formally Barcelona had a lot of time to save themselves and only one goal was needed for this (not very much given the home status of the game, right?)
As for the final, Real Madrid plays Osasuna there - probably the most difficult thing in such a game is to find motivation and not underestimate the opponent.