single.phpにタームの関連記事を表示する

<?php
    $terms = get_the_terms($post->ID, 'カスタムタクソノミーのスラッグ名');
    foreach( $terms as $term ) {
        $slug = $term->slug
    }
?>
<?php
$arg = array(
  'post_type' => 'カスタム投稿タイプのスラッグ',
  'posts_per_page' => 4,
  'orderby' => 'rand',
  'post__not_in' => array($post->ID),
  'tax_query' => array(
    array(
      'taxonomy' => 'カスタムタクソノミーのスラッグ',
      'field' => 'slug',
      'terms' => $slug
    )
  )
);

$wp_query = new WP_Query($arg);
?>

<?php if( $wp_query->have_posts() ): ?>
<?php while( $wp_query->have_posts() ): ?>
<?php $wp_query->the_post(); ?>
--- 繰り返し処理 ---
<?php endwhile; endif; ?>
<?php wp_reset_postdata(); ?>