<?php if (have_posts()) : ?>
    <?php  $count = 0; ?>
	<?php while (have_posts()) : the_post(); ?>
                    
					
         <?php if ($count % 4 == 3) { ?>
	     <div class="post last">
	 <?php } else { ?>
             <div class="post">
          <?php } ?>
                            
          <h2><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
          <div class="entry">
          <?php the_content(); ?>
         
          </div>
          </div>
                    
          <?php $count++;  ?>
     <?php endwhile; ?>
 <?php endif; ?>

Wordpress - Add last class to every 4th post

wordpress

PHP