Left nav not showing 4th level nav when 4th level page selected

[Collins]

Left nav is set up like this:

Main A Category
- Second A Category
-- Third A Category
--- Fourth A Category

Main B Category
- Second B Category
-- Third B Category
--- Fourth B Category

All the links look fine in the left menu, except when clicking on a Fourth Category page. When that's done, it goes to this:

Main A Category
- Second A Category

Main B Category
- Second B Category

How do you manage to display the entire directory tree in the left navigation when going to a Fourth Category page?