<?php

   $tags = wp_get_post_tags($post->ID);

   if ($tags) {
     $tagnumber = get_post_custom_values("tagnumber"); // Get the value/tagnumber from Custom field
     if (isset($tagnumber[0])) {
       $tagnumber = get_post_meta($post->ID, 'tagnumber', true); // If there is a value take it...
     }
     else {
       $tagnumber = 0; // ...otherwise take the first tag
     }                        

   $first_tag = $tags[$tagnumber]->term_id;
   $args=array(
    'tag__in' => array($first_tag),
    'post__not_in' => array($post->ID),
    'showposts'=>4, // Number of related posts
    'caller_get_posts'=>1
   );
   $my_query = new WP_Query($args);

   if( $my_query->have_posts() ) {
     echo 'Related Posts';
     while ($my_query->have_posts()) : $my_query->the_post(); ?>
     <a href="<?php the_permalink() ?>" rel="bookmark" title="Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a>

 <?php
 endwhile;
 }
 }
?>