I don't know if nobody else is as paranoid about this as I am (if so, I "blame" the quite-excellent spy-movie-style tension inherent through the piece, particularly the last "day"), but to me Boris' final action in this one very strongly hints that his orders from Moscow include "Terminate Natasha". The whole "silently screwing in the silencer" bit while her back is turned, with the emphasis that he's keeping his tone neutral, is tossing major red-flags for me.

I'm thinking that Natasha has not been quite so successful at keeping her affection for Mr. Stark from coming to the attention of her superiors as she had hoped, and that the loss of Prof. Vanko on her watch, -at Stark's shindig-, they're taking no chances.

Again, might just be me reading into it the wrong way, but I wouldn't be surprised if installment 5 starts out with a dust-up in a hotel room.

-Man- this one was fun.