Yes, she may well read a lot, but if she reads quickly to enjoy the story, she may not consider the spelling or punctuation.
I sometimes suggest DICTATING text to a tape recorder, or phone that records, then listening to it, making amendments if required, and finally writing it or typing on a computer. However, I advise this mostly for children who are reluctant writers, which is not the case here really. Nevertheless, she might like to give it a try, as being a different approach.
Books by Lynne Truss, and David Crystal, deal with spelling and grammar; Yr 4, she could probably cope with the adult books ("Eats, Shoots and Leaves" is the classic one), or both authors do children's books as well I think.
Stories by Anne Fine could be worth reading, and I particularly like "Diary of a Killer Cat" which is written as if the cat was telling the story, and the human characters are less prominent than the cat. Try to study WHY certain words, expressions, punctuation or plot details are used, from the cat's point of view.
Finally, what I often suggest is this book:
An inexpensive and easy to use book, that can encourage children with reading, spelling and writing, and really help them to understand Phonics, is reviewed in the MN Book Reviews section. Just search ‘Phonics’ and my name.