Pulling in Related Posts based on Post Type array

By default WP_Query() will query posts (unless tax_query is set). You can change this by setting the post_type key in your $args array.

To search for the same post type as is being currently viewed you can do the following:

$args=array(  
    'post_type'        => get_post_type(), //gets the post type of the current 'post'
    'tag__in'          => $tag_ids,  
    'post__not_in'     => array($post->ID),  
    'posts_per_page'   => 4, // Number of related posts to display.  
    'caller_get_posts' => 1  
);