Add Class After 4th Post

Just check $wp_query->current_post instead of using that mod operator.

if ($wp_query->current_post === 4) { ?>
    <div class="vender_hr"></div>
<?php }

But don’t use query_posts. Create a new WP_Query object. What you are doing is kind-of weird. You are checking to see if there are posts, then clobbering the main query which invalidates the have_posts check you just did, and then Looping though the post set. That really should look something more like this:

$qry = new WP_Query("posts_per_page=8&post_type=post&orderby=rand");
if ($qry->have_posts()) { 
  while ($qry->have_posts()) {
    $qry->the_post(); ?>
    <article id="post-<?php the_ID(); ?>" <?php post_class(); ?>> <?php echo eh_postImage(240,240,1); ?>
    <?php the_title(); ?>
    <?php the_content_rss('', FALSE, '', 60); ?>
    </article><?php 
    if ($qry->current_post === 4) { ?>
      <div class="vender_hr"></div><?php 
    } 
  }
} 

Also, the_content_rss has been deprecated since WpordPress 2.9. If you had debugging enabled as you should have while you are working, you’d see the notice.