Patch 1  -- preview of some edits to chapter 4.  Don't actually pull!!!  :-)

I decided to edit chapter 4 for my own sake -- as a way of cementing the stuff I (think I) have learned.  I think it's clearer with these edits, and I wanted someone from Orbis Tertius to look it over to see if it looked like an improvement to them as well.    Or, if I've gone badly astray anywhere, I'd appreciate the feedback.

As I said above, I have yet to finish editing the whole document, so the tone isn't consistent all the way to the end.  So I'm not actually requesting you to pull this, I'm just asking you to look it over.

If you think it's a good thing, let me know and I'll submit another pull request when I'm done.  Otherwise I'll keep it to myself.