I'm sorry you're having problems with the site. Sometimes units get "hidden" from students for various reasons.
Can you try going to the CSD page, choosing the student's section, then clicking "hidden" in the Unit 4 area. This will hide the unit from the entire class. Refresh the page, then click "visible" in the Unit 4 area. This should make the unit visible for all students, even the student that couldn't see it before.
If this doesn't work, please submit a bug report so we can troubleshoot with you.