Write a single-sentence summary of your book. Imagine that you are going to tell a fourth grader about the plot of the book. Focus on the hook, the most interesting thing about the book, and write a sentence based on that hook. This sentence will help keep you focused on your book's plot as you write. For this age group, the hook should be very clear and easy to understand, such as "a boy finds out his mean teacher is an alien."
Write an outline for your fourth grade book. Some writers prefer long, thorough outlines, while others find them constricting. You can use any method you feel comfortable with in this process, such as writing a one-sentence description of each scene on a notecard or filling out a complete detailed timeline of the plot as a spreadsheet. A fourth-grade book should have one main plot and no more than one subplot. It typically involves only one main character, rather than several. Books for fourth graders usually focus mostly on the main plot - for example, the boy's quest to prove his teacher is an alien - and spend perhaps 20 percent of the book on a related subplot, such as the boy's parent's concern that he won't pass fourth grade.
Write a rough draft one chapter at a time, using your outline as a reference. No matter how much you edit as you write, you will most likely need to do at least one revision, so allow yourself to write freely and without concern for mistakes and inaccuracies. The most important thing is the voice, which is the style you write in that represents your main character. A fourth-grade narrator can be funny, sarcastic, quirky or sweet but cannot be boring, or you will lose the reader. Avoid being preachy or trying to "teach a lesson" in your book, as elementary-aged students are very perceptive of this. Focus instead on writing an entertaining story.
Close your first draft when you finish, and do not read it for a few days. Writing a book takes a lot of time, and you become so involved in the story that it might be difficult to see the flaws. After a break, read through your book, and take notes on everything from grammatical errors to plot holes and other major problems. Read your book aloud and pay attention to the language, which should be natural for a fourth grader to speak and understand.
Revise your book at least once. When you feel you have a strong draft, ask both adult and fourth graders to read your book and give you honest feedback. Remember that a fourth grader's attention span is shorter than yours, and while your writing can be beautiful, it also has to be understandable and relevant to a fourth grader. Edit your book down to 30,000 words or less, as this is the ideal word count publishers look for in a book for fourth graders.