Review fourth-grade sentences for subject-verb agreement. This is one issue that plagues young writers. Make sure that singular subjects use singular verbs and plural subjects use plural verbs. This is usually not a problem with simple sentences, such as "He is my friend." When plural subjects are separated by a conjunction, kids are frequently uncertain as to what type of verb is needed. Indefinite pronouns, such as "everybody," often cause subject-verb disagreement in students' writing.
Look for pronoun confusion in fourth-grade writing. As with subject-verb agreement, this is often an issue that crops up when children begin to write long sentences. It is easy to keep pronoun and antecedent in agreement in a pair of sentences such as, "I like John. He is my friend." The pronoun "he" clearly refers to John. It is not uncommon to see a student write "I dropped the vase on my toe and broke it." What was broken -- the toe or the vase?
Check to see that students are using prefixes and suffixes correctly. Students may simply misuse them or overuse them. Spelling affixes correctly may also be an issue.
Review students' use of all eight parts of speech. In fourth grade, kids are moving on from very simple sentences to longer ones that try to incorporate more parts of speech. They move on from describing things with adjectives and begin to describe actions with adverbs. They might not always do this properly.
Correct spelling and punctuation. All of the new words in their vocabulary are growing in length and complexity. Also, as their sentences grow longer, students may become confused by punctuation or just plain forget it.