The concepts described in Chapter 4 correspond to the key notations in class diagrams. Those concepts are the first ones to understand and become familiar with, as they will comprise 90 percent of your effort in building class diagrams.
The class diagram technique, however, has bred dozens of notations for additional concepts. I find that I don't use these all the time, but they are handy when they are appropriate. I'll discuss them one at a time and point out some of the issues in using them. Remember, however, that they are all optional, and many people have got a lot of value out of class diagrams without using these additional items.
You'll probably find this chapter somewhat heavy going. The good news is that during your first pass through the book, you can safely skip this chapter and come back to it later.