Custom Post Type – Styling Posts Differently

Having trouble getting a solution for this. I have a custom post type and I need to style the posts differently.

The first post should have its own class, the next three posts should have their own class, and then the rest don’t need a unique class.

<div class="first-post">POST CONTENT</div>
<div class="second-post">POST CONTENT</div>
<div class="third-post">POST CONTENT</div>
<div class="fourth-post">POST CONTENT</div>
<div>POST CONTENT</div>
<div>POST CONTENT</div>

Here is what my code looks like right now:

<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>

<?php
$destinations = get_posts(array(
	'meta_query' => array(
		array(
			'key' => 'destination',
			'value' => '"' . get_the_ID() . '"',
			'compare' => 'LIKE'
		)
	)
));
?>

<?php if( $destinations ): ?>
<?php foreach( $destinations as $destination ): ?>
<?php $post_class = ('alpha' == $post_class) ? 'omega' : 'alpha'; ?>
<?php $photo = get_field('main_image', $destination->ID); ?>

<div class="<?php echo $post_class; ?>">
	<div <?php post_class($style);?> id="post-<?php the_ID();?>">
		<div class="post-entry">
		<a href="<?php echo get_permalink( $destination->ID ); ?>">
			<?php echo get_the_post_thumbnail( $destination->ID, 'grid-thunb' ); ?>
		</a>

		<h2 class="post-title"><a href="<?php echo get_permalink( $destination->ID ); ?>">
		<?php echo get_the_title( $destination->ID ); ?>
		</a></h2>

		<p><?php $customLength=20; echo get_the_excerpt(); ?></p>

</div>
</div><!-- end of #post-<?php the_ID(); ?> -->
</div><!--end of post class -->
<?php endforeach; ?>
<?php endif; ?>

Really appreciate the help! Thanks!