posts_per_page not working on custom post

Hi,

I am using the following code to try and call 4 random custom posts (issues), but for some reason it is showing 5 (there are only 5 atm) and they are in date order;

<?php $args = array('post_type' => 'issues', 'post_status' => 'publish', 'posts_per_page' => 4, 'orderby' => 'rand', 'caller_get_posts'=> 1);
$my_query = null; $my_query = new WP_Query($args); if( $my_query->have_posts() ) {while ($my_query->have_posts()) : $my_query->the_post(); ?>
<div class="col-xs-12 col-sm-6 col-md-3 newsWrap">
<div class="newsPosts">
<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>">
<?php the_post_thumbnail('thumbnail'); ?>
<h4 class="heading"><?php the_title(); ?></h4>
</a>
</div>
</div>
<?php endwhile; } wp_reset_query(); ?>

Any advice would be very helpful, thanks!