I'd like to insert Google DFP ads every fourth post on my

I’d like to insert Google DFP ads every fourth post on my homepage. I’m using the following code to do so:

<?php

if(($row_count % 4) == 0){?>

<div class='row dd-feed-ad'>

<div class='col-md-12' id='div-gpt-ad-xxx-0' style='width:480px; height:60px;'>

<script type='text/javascript'>

googletag.cmd.push(function() { googletag.display('div-gpt-ad-xxx-0'); });

</script>

</div>



</div>

But that means that one specific ad would repeat. Does anyone know how I could set it up so there is different content after post 4, post 8, post 12, etc?