Related Posts with removing some categories

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You can use the category__not_in parameter to exlude categories.

get_posts uses the WP_QUERY parameters

Your code will be :


$related = get_posts( array( 
               'category__in' => wp_get_post_categories($post->ID),
               'numberposts' => 5, 
               'post__not_in' => array($post->ID),
               'category__not_in' => array(1,2,3,4)'
          ) );