Ah, right, that goes back to my comment that all of the fourth level subcats are inside first level subcats. When th first level is hidden, it takes the fourth level with it.

I had an idea for using height or line-height...
Code:
* html #categories li.cat-subs {height: 0;}
and only if you need it
Code:
* html #categories li.cat-products {height: 30px;}
Let me know how this works (I wish there were web developer tools for IE6).