We're reading the Magic Treehouse with Leif. It's perfect for him. At three/four, he's ready for stories with more plot/story, but still wants pictures. These chapters are short, and have a good arc to them, and there are pictures throughout the book. When we have a little time we read one or two chapters, but we can make it through a book in an evening if he wants to. At his age, we change a few lines (eg "I'm going to kill you" to "I'm going to hurt you") but for the most part they're exactly on target. We suspect he'll want to reread the series when he can read for himself, but for now these are great for bedtime reading.