We've used the Challenge Math books in the past. They are very cute with cartoon characters who introduce the concepts. I do think that they are good books that put things in real world context and are written to be comprehensible by kids. We started with his Ten Things All Future Mathematicians and Scientists Must Know book to get my dds interested in knowing how this info would be applied in real life and then did Primary Grade Challenge Math. For a 4th grader, you will probably want the Challenge Math, not Primary Grade Challenge Math, though.
