3 columns on main page, 4 columns on category pages

[vandis]

Hi,

I want to display 3 columns of categories on the main page of my directory
But on the category pages, I wish to have 4 columns instead of 3.

i tried this
{if $category.ID gt 0}
{php}$this->assign('CATS_PER_ROW', '4');{/php}
{/if}

but it doesnt seem to work..

any sugestions?
thank you for your time