Adverts between post.

Hi Guys,

Can anyone can help to insert correct code into my index.php to place google ads under every 4th posts? Is it possible to do it? Below you can find me index.php

<?php
/**
* The main template file.
*
* This is the most generic template file in a WordPress theme
* and one of the two required files for a theme (the other being style.css).
* It is used to display a page when nothing more specific matches a query.
* E.g., it puts together the home page when no home.php file exists.
* Learn more: http://codex.wordpress.org/Template_Hierarchy
*
* @package Silk
*/

get_header(); ?>

<div id=”primary” class=”content-area”>
<main id=”main” class=”site-main” role=”main”>

<?php if ( have_posts() ) : ?>

<?php get_template_part( ‘loop’ ); ?>

<?php else : ?>

<?php get_template_part( ‘content’, ‘none’ ); ?>

<?php endif; ?>

</main><!– #main –>
</div><!– #primary –>

<?php get_sidebar(); ?>
<?php get_footer(); ?>