<?php if ( have_posts() ) : query_posts('cat=4');
      while (have_posts()) : the_post(); ?>

      <?php the_post_thumbnail(array()); ?>
            <span class="advantage"><?php echo get_post_meta($post->ID, 'advantage', true); ?></span>

                  <a href="<?php the_permalink(); ?>"><span class="course-title"><?php the_title(); ?></span></a>
                     <span class="teacher-name">
                           <?php echo get_post_meta($post->ID, 'teacher', true); ?>
                     </span>

      <?php endwhile; ?>

 <?php endif; ?>
 <?php wp_reset_query(); ?>