Usually a math book includes text, examples and questions. They are mixed up together. You learn the information from the text and examples, then you test your learning by answering the questions (solving the problems). If the questions (problems) are too hard, you need to go back and re-read text and examples (you need to study) until you understand.
To "do a chapter" implies doing all of those actions. Not just questions (problems), not just reading, not just learning.
Edit: Another kind of book is a novel. All you do is read it. We would not say "do a chapter" about a novel. We would say "read a chapter".
If all you are doing is reading chapter 4, we say "I am reading chapter 4." We don't use "do" to mean "read".
But you may be doing something else. Are you studying it? Are you taking notes about it, for a report? Are you analyzing it for a school assignment? There may be other things you can say, in addition to "read".