Not that anyone is already thinking about the end of the year, but these are the companion books you might want to buy to take advantage of the buy 3, get 1 free offer Amazon has in their 4-for-3 promotion. The Big Test is all about the class getting ready for their state test (I think we can all relate to that one!) and Last Day Blues shows the students preparing to say goodbye.
If you need a good recommendation for your fourth book, I recommend How to be Cool in the Third Grade by Betsy Duffey. It's my first whole class read aloud just about every year. It not only has a good lesson, but it's packed with lots of laughs too.