I would use these writing prompts to have students practice expository writing. I would write at the top of the sheet "Write to explain..." and let the students choose their writing prompt.
Use this rubric to easily and consistently evaluate your students' expository writing. $1
Expository writing worksheet that helps students generate ideas for their paper.
Expository Thinking Map
Expository Essay Format freebie in Laura Candler's Writing File Cabinet
Adventures of a Future Teacher: Expository Writing Anchor Chart elementaryeducati...
Expository Writing Unit
This is cool, but I think a touch too much for 4th graders. Middle school writing, I think.
Expository Writing Anchor Chart | Expository vs. Narrative Compositions
Expository Writing Hooks Anchor Chart