Interacting with css class

Hi folks, I have no idea how to write this in javascript…

I created a navigation in css and I want to be able to hide the first menu’s sub items once the user hovers over the other three menu items.
this is the preview link:
http://www.1826db.com/prototype/sukgyung/listnavTest.html

Additional info:
The css code for first menu item is:
#nav li.firstItemOn ul {
display: block;
border:none;
margin-left:-16px;
}

The other three menu item names are secondItem, thirdItem, fourthItem.

Anybody know how to disable a css class once you hover over those three menu items?

Thanks in advance.