Preparation is the key to doing well on tests. Help your child develop good study habits. It is especially important for your fourth-grader to learn to review frequently. Redoing assignments can really cement knowledge in children's minds. Also, help your child get in the habit of doing all the exercises in the end of the chapter materials in textbooks. They are a great preview of most tests. Then when test time arrives, your child will be seeing a lot of familiar material, which will make her feel more confident.
You are going to have to try to de-emphasize the importance of tests - not an easy task. When she gets home from school, don't ask your child how well she did on a test; talk about her day. If you discover that your child didn't do well on a test, try not to appear worried or anxious. Instead, make comments like: "You'll do better next time. Tests don't measure everything you know. You really tried."
Your encouragement can build your child's self-confidence. Before tests, make comments like: "You really prepared for the test. You'll do well." Also, be supportive of your child's efforts to prepare for tests by being willing to quiz her on spelling, vocabulary words, or the multiplication tables. Finally, teach your child to focus on being pleased with the questions she can answer.