Well, the last step took me more time than expected! Partly this was just life stuff intervening but partly it has to do with the fact that there are a bunch of example games between chapter 3 and chapter 4 and then again at the end of chapter 4. These example games are harder to bring myself to work through. I think the reason for this is that the text will often make sense to me, but the examples will remain obscure, so there's less sense of reward for my effort. Of course, I do my best to work through the variations and understand what's happening as well as possible, but many of the moves still seem mysterious. That being said, I do feel I am learning from the example games as well.
Chapter 4 is also a very meaty chapter. It is an important topic and he's presenting many ideas about common situations that take some thought to digest.
One practical point that I thought quite useful was the rule that when you have a possible passed pawn, then it should lead the advance.
As far as the question of blockading. I think I get the basic ideas here. The main idea is that the pawn wants to advance. As a result it is necessary to blockade it. It makes sense to me that the square in front of the passed pawn can become a weak square due to the fact (among others) that the pawn itself shields the blockader. Furthermore it makes sense that the blockaded pawn ties up other pieces who are invested in it's fate. I am also clear on the fact that there are various considerations about the piece which blockades and that it's function is not merely limited to blockading but that it should be able to make threats as well.
He also takes some time to do a diversion into basic endgame material. Here he scorns the notion of opposition and insists on the priority of the concept of flanking. Silman seems to have sort of synthesized the two concepts because he uses the notion of the opposition but explains it in terms of outflanking. Perhaps when Nimzowitsch was writing, opposition was less well explained and treated more as a kind of magical quality.
I guess the parts that I'm not so clear on are the questions of when precisely it is appropriate to advance the pawn. Here of course he gives a schema that is supposed to outline the issues, but it is very general. Also the question of when it is appropriate for a passed pawn to sacrifice itself for the activity of the pieces behind it. I guess these would come under the idea of the "the lust of the passed to pawn to expand" as N describes it. I feel like I have a rough concept of the idea. Obviously most basically the pawn wants to queen and so advancing toward that goal is a main driving force of play.
Anway, as I mentioned, it seems ok that some of the material is over my head. At least I'm getting acquainted with the ideas and hopefully over time the ideas introduced will have a chance to grow.