op he has a processing and ordering issue, and it will take time to get the ordering in place. Modelling will work, but it will take a long time.
Basically your DS needs to understand that in English we have a very basic simple sentence structure. It is called SVO- *subject (person,place,thing-usually a noun) verb (a doing word) and then object- (person, place,thing, feeling-usually a noun)
I think if you want to start with working on his word order you need to begin with Simple Questions that prompt simple 3/4 word SVO answers-
For Example
(you say)What do you want lunch for lunch?
(you model)I want a ham sandwich
you prompt DS, (DS says) I want a ham snadwich
You could make up some nice picture cards that make silly sentences and then good sentences.
4 is very young. What was his diagnosis?