I sort of assume when they get to fourth grade that they are familiar with the parts of a textbook and that is not a good assumption to make.
The jump from second to third grade is quite strenuous academically and initiating a lesson within the first couple of weeks of school on the parts of a textbook might actually be quite helpful in the long run. I proposed that they put together an interactive white board lesson (we use Promethean in our district), have the students go through the parts of the textbook as they go through the lesson, have a higher level textbook (grade 5 science) on the desks as well so that they see that the "parts of a textbook" apply to all grade levels, and then have them conduct a textbook scavenger hunt with their third grade science textbook.
I was looking online for some text to use that went over the parts of a textbook and cobbled together a LH page assignment that you can download HERE (please feel free to modify it, as it really was pieced together). The RH assignment is a scavenger hunt using our textbook, which I didn't include as a download since our textbook might differ from those used by others. I could have made the scavenger hunt longer but most of the third graders I have been talking to get between 30-40 minutes to teach science and I thought a one page scavenger hunt was doable with that time frame (including the lesson).