I'd say A Certain Small Shepherd is more of a picture book, but has pretty good/strong vocabulary. smile You might want to be aware that it's a religious book (I don't want anyone to be offended or surprised) and that the beginning is pretty sad, especially for a four-year-old, and that for modern graphic novel fans it might be too slow. But if your son is mature, I think it's s good fit.
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Happiness can be found even in the darkest of times, if only one remembers to turn on the light.