How to display posts of specific category using a custom Query in WordPress?

the recommended way is:

    <?php
    $query = new WP_Query('category_name=Category&posts_per_page=4');
    if($query->have_posts()) : while($query->have_posts()) : $query->the_post();
    if (has_post_thumbnail()) {
        ?>
            <a href="https://wordpress.stackexchange.com/questions/162985/<?php the_permalink(" ') ?>" title="<?php the_title(); ?>"><?php the_post_thumbnail(); ?></a>
        <?php
    }
    ?>
    <h2><a href="https://wordpress.stackexchange.com/questions/162985/<?php the_permalink(" ') ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></h2>
    <?php
    the_excerpt(); // or the_content(); for full post content
    endwhile;endif;
?>