Why should they show? When you click on the c1 , you're hiding the other 2 but there is no code to make c4 and c5 appear. Am I missing something? Where is the code that should make c4 and c5 appear?
This line of code is only getting the elements for C1,C2 and C3. I don't know if you expect C4 and C5 to be in that array as well.
this.ch = box.getElementsByTagName('DIV');
UPDATED:
I don't know where you want to fit this into your logic but here is how you would show/hide C4 and C5 with just javascript.
Show the elements -
document.getElementById("C4").style.display = "block";
document.getElementById("C5").style.display = "block";
Hide the elements -
document.getElementById("C4").style.display = "none";
document.getElementById("C5").style.display = "none";