Have you considered Poetry? There are some great ones for 4th grade. Lots of inference material. Then you could select/prepare an inference question that suited all the teachers in the grade level. Some poetry books have the q's included within the book.
Lewis and Clark expeditions are two other stories that you could use. Both are suitable for inference assessment q's. Inference per chapter. Inference whole story. Maybe certain selected pages of the chapter, as in a fluency/comprehend read.