I think you're spot on with a lot of this. Bobby, I'm very afraid, is Marked for Death...and yeah, his death is going to be the catalyst for either Sam vs. Dean, or the resolution and coming together of the boys after Sam vs. Dean.

Also agree that Sam's going to have Lilith's head, and that he's going to turn out to be good in the end.

I think S4 is going to end with the boys just having resolved their conflict, but failing to save the final seal (possibly because one of them has refused to kill the other at the very last moment). And the Big Bad is going to rise. That's the cliffie we'll be left with over the summer.

I hope! Because really, all of that sounds pretty freaking AWESOME!! :)