krawe on "Limiting categories via dropdown"

I have this code. How can I instruct the drop-down to not show certain categories in it [say cat=-4 or something like that]:

<?php wp_dropdown_categories( 'tab_index=10&taxonomy=category&hide_empty=0' ); ?>

Any ideas? I don't want to break what works, but I have some categories in the dropdown I don't want visible.

Thanks!