kiyakoiya, unless I missed it, it never said anywhere that the Section 4 reading is from Problems in Contract Law, but that's what I assumed.
The first chapter of the assigned reading is called "An Introduction to the Study of Contract Law" and the second chapter is called "Enforcing Promises: Bases of Legal Obligation". Maybe you could check those titles with what's in the casebook.
The relevant pages were included w/ the section letter in xeroxed form (without any markings to indicate what book they were taken from, as far as I could tell). I'd also guess that they're from the case book, but I'm not 100 % sure.