Showing related post for custom taxonomy

Try this

  $tags = wp_get_post_tags($post->ID);
  $tag_ids = [];
  foreach($tags as $individual_tag){
     $tag_ids[] = $individual_tag->term_id;
   }

   $args=array(
      'tag__in' => $tag_ids,
      'post__not_in' => array($post->ID),
      'posts_per_page'=> 4,
    );
    $my_query = new WP_Query( $args );
    
   //Check
    echo '<pre>';
    print_r($my_query);
    echo '</pre>';