Standard Loop:
<?php while ( have_posts() ) : the_post(); ?>
<?php get_template_part( 'content', get_post_format() ); ?>
<?php endwhile; ?>
Loop checking for 4th post:
<?php $i = 0; while ( have_posts() ) : the_post(); $i++; ?>
<?php if(!$i%4): get_template_part( 'content', get_post_format() . '-4th' ); ?>
<?php else: get_template_part( 'content', get_post_format() ); endif; ?>
<?php endwhile; ?>