Related posts from primary category

related_posts_category.php

<!-- Posts in primary category -->
<div class="more-posts-after-content row">
<?php
global $post;
setup_postdata( $post );
?>
    <?php
    // Check for current post category and add tax_query to the query arguments
    $cat = new WPSEO_Primary_Term('category', get_the_ID());
        $cat = $cat->get_primary_term();
        $catName = get_cat_name($cat);
        $catLink = get_category_link($cat);
        $catId = get_cat_ID($catName);
        $args = array('cat' => $catId,
        'posts_per_page' => 4, // How many items to display
        'offset'            => 6,
        'post__not_in'   => array( get_the_ID() ), // Exclude current post
);
    // Query posts
    $wpex_query = new wp_query( $args );

    // Loop through posts
    foreach( $wpex_query->posts as $post ) : setup_postdata( $post ); ?>

    <article class="col-md-3" id="post-<?php the_ID(); ?>" >
      <?php if ( has_post_thumbnail() ) : ?>
          <a class="post-teaser" href="<?php the_permalink() ?>" title="<?php the_title(); ?>">
            <div class="thumbnail-block"><?php the_post_thumbnail('medium'); ?></div>
            <div class="title-block"><span class="h3 title-for-widget"><?php the_title(); ?></span></div>
          </a>
      <?php endif; ?>
    </article>
  <?php
  // End loop
  endforeach;

  // Reset post data
  wp_reset_postdata(); ?>
</div>
<!-- End of more posts in primary category -->