Add somethign before and after every group of 4 posts...

gistfile1.aw

<?php $tombstones = new WP_Query('category_name=tombstones&posts_per_page=-1'); ?>
 <?php $counter = 0; ?>
 <?php  if( $tombstones->have_posts()) : ?>
  <?php while ( $tombstones->have_posts() ) : $tombstones->the_post(); ?>
  <?php $counter++; ?>

  <?php if ($counter % 4 == 0) echo '<div class="slide"><h1>Beginning</h1>'; ?>
  <?php the_content(); ?>
  <?php if ($counter % 4 == 0) echo '<h1>Ending</h1></div>'; ?>
									
<?php endwhile; endif;?>
<?php wp_reset_postdata(); ?>

Try this

<?php 
$tombstones = new WP_Query('category_name=tombstones&posts_per_page=-1');
$counter = 0;
if( $tombstones->have_posts()) :
while ( $tombstones->have_posts() ) : $tombstones->the_post();
    if ($counter % 4 == 0) echo '<div class="slide"><h1>Beginning</h1>';
    the_content();
    if ($counter % 4 == 0) echo '<h1>Ending</h1></div>';
    $counter++;                             
endwhile; endif;
wp_reset_postdata(); ?>