Level 3 offers view only on categories and level 4 jumps straight to the full whack (add, edit and delete).
I am writing themes using category queries. Users adding categories cause no problems, deleting them on the other hand can break an entire site.
It would be nice to have more control over users and categories, so they can only add, or add/edit.
There seems no way of protecting / locking a category from deletion on any user level 4 or above, whether it was created by a user of a higher level or not.