Chapter 4 edit

[hazel-nut]

here are the open issues for chapter 4.

blockers for this PR:

• 27 is the biggest remaining TODO; i didn't edit the section in question.

• 29 is an open question i wanted to check on.

• 30 is a nice to have but i think should get included here.

[jonschwenn]

Chapter 4 has been fully polished, tested, and all open issues have been resolved.

The only outstanding item I am still working on is to add in diagrams which I will slip in when they are ready.

If all the edits look good to you @hazel-nut -- then Chapter 4 is "done".

[hazel-nut]

@jonschwenn , this repo's protected so i can't merge :woman_shrugging: but go ahead whenever you're ready