Remove top level parent category from permalink

Hi there,

I am stuck on finalising my permalink structure and can’t seem to find a solution. I have my post in the forth tier like this .com/cat1/cat2/cat3/post/

Each category is a child of the one above so that I can contain a loop of the categories below. Plus I am happy to have the last category URL .com/cat1/cat2/cat3/

But what i really want is to remove the top level parent from the final URL so that my post is only in the third tier of the structure.

I have it set to;
/%category%/%postname%-%post_id%/

But it automatically puts all categories in the URL. So my question is really how can i remove the top level parent category from this structure? I hear there are some ways to do a re-write but cants seem to find out how. Is there are way of doing it without touching the core WP files? i would of course prefer that to avoid the hassle of when i upgrade…

Or is there a plugin? I found one plugin (sorry forget the name now, i think it was advance permalinks or something) but i heard it wasnt stable.

I would be very grateful if anyone had any ideas or suggestions to help me out ??

Many thanks in advance.