In fourth grade student can begin to learn how to develop well written essays.  A great way to know what your 4th grader needs to learn is by seeing examples of writing. 

Also, take a look at some 5th grade writing samples. 

Fourth Grade Writing Samples

• Scored 4th Grade Writing Samples From Oregon Department of Education
• Utah Education Network 4th Grade Narrative Writing Samples
  4 graded writing samples