Ann-Marie,
They're actually not different editions of WriteShop I and II, just different covers/formats. We've simply stopped offering the binders mostly due to skyrocketing production costs, but also to give the books a fresh look.
From the Teacher's Manual product description: http://writeshop.com/product/teachers-manual-writeshop-i-ii/
Teacher’s Manual Has a New Look!
The Teacher’s Manual’s new format offers a soft cover and plastic-coil binding.
Sections feature sturdy tabbed divider pages.
The text has not changed. You will find the same 4th-edition content.
It is compatible with all 4th-edition WriteShop I and WriteShop II Student Workbooks.
Hope that helps! If I can answer any other questions, please let me know.