I assume "it" is a certain book here. If I were a teacher assigning some reading, I wouldn't say "read the book starting from chapter 4 if the student has already read chapter 3. Then the student isn't starting the book from chapter 4, because they started at the beginning, or at least from chapter 3. I would just say, "Since you've already finished Chapter 3, please read Chapter 4 [or Chapters 4 through 6] for next time." Or you could say, "Since you've finished Chapter 3, please continue reading from Chapter 4."