The best thing to do to introduce writing to 4th graders is this:
First introduce the concept of writing as a circular process
and also emphasize the revision step. (If I had a nickel for
every time a kid told me they were done; I'd be rich.) A good
book for this is "What Do Authors Do?" by Eileen Christenlow.
It is a good lesson for the first week. Chart the steps the
author in the story does as a whole group, then make a writing
process sign to post in class. Check assessment by having
students create a circular process chart for something they do
all the time ex: make toast, a bowl of cereal, get ready for
school.
The second week of school slowly introduce the 6 traits: maybe
two a day. Find examples of each trait in familiar books.
Devise mental models so students can associate each trait with
something in their mind. Nothing too in depth, but something
short and sweet so they are aware of the traits and some
characteristics of them.
Once they have the writing process and are aware of the
process, I move to idea. I spend 4 to 6 weeks on idea. If
they don't have idea down, they no introduction will make it
shine. While teaching traits, I am doing grammar (sentence
structure and editing) I also have a good writers chart so the
class can remember features of good writing (zoomed in, sensory
detail, catchy introduction, etc)
I don't know if this helps or hurts you. Although I have
Harcourt Collections at my school. I use trade book whenever
possible to teach reading and writing.