<?php
// Default arguments
$args = array(
	'posts_per_page' => 4, // How many items to display
	'post__not_in'   => array( get_the_ID() ), // Exclude current post
	'no_found_rows'  => true, // We don't ned pagination so this speeds up the query
);

// Check for current post category and add tax_query to the query arguments
$cats = wp_get_post_terms( get_the_ID(), 'category' );
$cats_ids = array();
foreach( $cats as $wpex_related_cat ) {
	$cats_ids[] = $wpex_related_cat->term_id;
}
if ( ! empty( $cats_ids ) ) {
	$args['category__in'] = $cats_ids;
}

// Query posts
$wpex_query = new wp_query( $args );

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

	<a href="<?php the_permalink(); ?>" title="<?php echo esc_attr( the_title_attribute( 'echo=0' ) ); ?>"><?php the_title(); ?></a>

<?php
// End loop
endforeach;

// Reset post data
wp_reset_postdata(); ?>