I had the same question, and emailed the author. I have a 9 yo ds and a 6 yo dd. Here is her reply:
"Your question is a good one and the answer depends on what you want for your 4th grader. Some children at that age really want to learn to draw more realistically. This can be shown by a sudden dissatisfaction with their work as it is or they may request more in depth knowledge or they may not express such things. Anyway the 4-6 Book 1 answers the question, "How can I draw better?" It is primarily a drawing book with just enough technical information for that age. However, many people decide to use one book for their younger child and the older one (especially 4th and 5th graders) in order to instill a joy of art in their children. The K-3 Book One, which is what I definitely recommend for the 1st grader, is a great introduction to art in general, does not talk down to the student so that it feels "babyish", and lets the student work with a variety of media: drawing, painting, paper art, and clay. People have commented that even their 7th graders enjoyed the projects and learned things from it (although the student art samples in it are drawn by 1st - 3rd graders, the projects are fun for any age.) Basically, I suggest the K-3 Book One for the younger. The older can work in that the first year together or move directly to the drawing book (Grades 4-6 Book One). If you feel she would have fun doing a variety of projects, and do the K-3 together the first year, your oldest would still have two years to go on and finish the 4-6 Grade books one and two. Note: There is nothing in the K-3 program that is a prerequisite for doing the 4-6 grade levels or any of the other levels so the option to jump in the middle is an option that we designed into the program. "
We decided to go with book 1 for both of mine. Hope that helps!
Ashley
www.startemiusacademy.blogspot.com