Writing definitions of words and phrases can be a useful way for students to practice their descriptive writing, while also improving and cementing their understanding of new words and concepts. After each student has written his definition of a certain word, refer to the dictionary entry of that word with the student and see how close he came to matching the description. For additional fun, have each student try to guess what word the other student was trying to describe, simply by reading definitions.
Writing instructions is a practical writing exercise which can have useful implications in later life, as it's likely every student will need to produce clear and logical writings at some point in the future. The instructions can be about anything, although it's especially helpful if they can immediately be applied; for example, if the instructions relate to the rules of a sport. To demonstrate the difficulties and ambiguities in writing instructions, ask the students to write instructions about their favorite way to dance, then giving the instructions to another student to perform. This can be entertaining, but can also underline how instructions can be interpreted in several different ways.
Book reports are a popular writing assignment for fourth graders, as this task will not only develop their knowledge of literature, but will also assist them in developing appropriate ways of describing information they've encountered. Encourage each student to write a report about a book she has recently read, discussing an outline of the book's story and talking about the message or meaning of the work.
A class newspaper provides an exciting opportunity for students to incorporate writing exercises into a real-life format. As a class, decide what sections the newspaper should include; for example, news, sport and features. Once the sections are decided, split the class into different groups for each section and ask each student to write an article in his assigned section. After the articles are finished, use desktop publishing software to assemble them in the appearance of a newspaper. Print a copy for each student.