How to tarteg post before last

Hi there, im using this code to target last post from a category, but i want to target post before last!

<?php $my_query = new WP_Query('cat=4&showposts=1');
  while ($my_query->have_posts()) : $my_query->the_post();
	$do_not_duplicate = $post->ID;
	$postid = $post->ID;
	?>
	<?php echo get_post_meta($post->ID, 'mainvideo', true);?>
         <?php endwhile; ?>