I like turning off the lights and letting their pupils dilate and then flipping them back on and hearing all the "AHH!" reactions when they watch each other's iris adjust. Teachers have to get their kicks somehow. To bring it all together, I have them complete a writing assignment on how the iris and pupil work together and then we illustrate it with an eye diagram. Of course we review our key vocabulary: sclera, cornea, iris, and pupil (it's just a hole!). I put an emphasis on our "4th grade sentences" and writing skills.
Here's an example of our final product:
Find my printables for this lesson at my TpT store.
We haven't even gotten to the inside of the eye yet! Sometimes I struggle to find resources for 4th grade science that aren't too elementary for my students. I love using AIMS resources for teaching the eye and the ear. I use the Human Body book for 5-9th grade even though I teach 4th because I think it has great resources, activities, and uses appropriate content vocabulary in an easy to read format. It has a digital format, or you can buy the book with CD for the files.