Get topmost category parent

I made it work with the following function:

function in_parent_category($cats, $_post = null)
{
	foreach ((array) $cats as $cat)
	{
		$descendants = get_term_children((int) $cat, "category");
		if ($descendants && in_category($descendants, $_post))
			return true;
	}
	return false;
}

Just feed the function with the category ID of the parent, and then it checks the current post (if in the loop) if it’s in a category which is a child of the category ID you specify.

if (in_parent_category(4))
{
	// do this
}