In terms of the details of the games and the matchup, I noticed something very key in Game 4. Each team has simplified its offensive game by sending pucks and bodies to the net on a more consistent basis.
Coaches call it shooting for opportunities.
I think it was a more important development for the Blues because it showed progress in them figuring out Rask.
They got two goals in Game 4 with defenseman Alex Pietrangelo simply shooting to create rebounds. If they want to score goals against Rask and win the Cup, they'll have to keep it up.
Like many great Finnish goaltenders, Rask has such a good glove hand. He gobbles things up and no rebounds are usually available.
I have noticed a pattern of the Blues, and Pietrangelo in particular in Game 4, going to the blocker side with their shots. It might go in, but if not, at least it can generate a weak-side rebound. Rask can't catch it, keep it and then have Boston's centers win the face-off, which kills any Blues momentum.
In the third period of Game 4, Pietrangelo went blocker side on him and Ryan O'Reilly was in the right spot to put in the rebound that was available.
Boston is so good at defending the slot, it's essential to put the puck on net in such a way that rebounds might just be there.
It even applies to bad angles, how the Bruins got their first goal in Game 4 from Coyle, when Chara's rebound was there for him.