Oh man! I typed up a list of strategies and then re-read your post. It seems you want a service to tutor and give personalized feedback. That sounds like a great business for a homeschooling mom.
I already made my little list, so, here it is-
The "writing with skill" book review suggests summarizing a short story, then building an outline off of the summary, then building a story off of the outline. This aligns with the common practice of having students illustrate stories in artwork, act out stories in skits, and summarize stories in book reports.
They based that off the book that said Benjamin Franklin wanted to learn how to write as well as the magazines he was reading, so he turned an article he read into poetry then wrote a short story based on the poem. By the time he got to the short story he'd practically forgot the original article.
I also enjoyed reading the sample chapters on amazon by Stephanie Harvey "strategies that work; teaching comprehension to enhance understanding". There were some good ideas in the sample chapters, which you can download to kindle.
I don't know if diagramming sentences is still in fashion.
Looks like this link has online games for any part of 4th grade LA skills you may want. http://www.internet4classrooms.com/skills-4th-langbuilders.htm
