Display featured image as thumbnail?

I am currently trying to display a post to my homepage from a category, along with an image (featured image).

so far, I have this code which grabs the post and displays it on my homepage:

    <div class="row">
  <div class="col-md-6">
    <?php query_posts('cat=4');
    while (have_posts()) : the_post();
    the_content();
    endwhile;?>
  </div>

This does display, but I have now attempted to set a featured image aswell.

I would like it to display like:

I have seen some suggestions such as

<?php the_post_thumbnail(); ?> but i'm not 100% where to add this.

Super thanks in advance!