Last post in row have different style

Hello, how to do that last post per row have different style?

<?php get_header();  ?>

<?php
// Show a selected number of posts per row
$posts_per_row = 4;
$posts_per_page = 8;
$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
$args = array(
   'posts_per_page' => $posts_per_page,
   'paged' => $paged,
);
query_posts($args);
if (have_posts()) {
   while (have_posts()) {
      the_post();
      if ((++$post_counter % $posts_per_row) == 1  || $posts_per_row == 1) {
         if ($post_counter > 1) {
            echo "</div><!-- End of post_row -->\n";  // End previous row
         }
         echo "<div class='post_row'>\n";  // Start a new row
      }
      echo "<div class='post_class'>\n";  // Start one post
         // Output post data here
         echo '<h2>';the_title();echo '</h2>';
         the_excerpt();
      echo "</div><!-- End of post_class -->\n";  // End of post
   } ?>
   </div><!-- End of post_row -->
   <div style='clear:both;'></div>
   <div>
      <div><?php next_posts_link('&laquo; Older Entries') ?></div>
      <div><?php previous_posts_link('Newer Entries &raquo;') ?></div>
   </div>
<?php } else {
   // Code for no posts found
}
?>
<div style='clear:both;'></div>
<?php get_footer(); ?>