Jekyll every 4th item in loop display differently - html

Question

I am using jekyll to populate my blog page with blog posts. I want there to be two posts for every div with the "row" class, but I also want every 4th item to be an ad (skip over the post and move to the next row but still include something else that's not a post).
So if there are 6 posts, the output should look like this
<div class="row"> <!-- 1st row -->
<div> {{ post.title }} </div> <!-- 1st post, no skip -->
<div> {{ post.title }} </div> <!-- 2nd post, no skip -->
</div>
<div class="row"> <!-- 2nd row -->
<div> {{ post.title }} </div> <!-- 3rd post, no skip -->
<div> THIS IS NOT A POST </div> <!-- skip post 4, put something else -->
</div>
<div class="row"> <!-- 3rd row -->
<div> {{ post.title }} </div> <!-- 4th post, because last item was skipped to display something else -->
<div> {{ post.title }} </div> <!-- 5th post, no skip -->
</div>
<div class="row"> <!-- 4th row -->
<div> {{ post.title }} </div> <!-- 6th post, no skip -->
</div>
<!-- and so on, so every 4th item is not a post, but the posts continue after the skipped post -->
I have the post loop part but I cant figure out how to add the SKIP
{% assign rows = site.posts.size | divided_by: 2.0 | ceil %}
{% for i in (1..rows) %}
{% assign offset = forloop.index0 | times: 2 %}
<div class="row blogitems">
{% for post in site.posts limit:2 offset:offset %}
<div class="col-md-6">
<p>{{ post.title }}</p>
</div>
{% endfor %}
</div>
{% endfor %}

Answer 1

I think you can just walk through your items while checking the modulo. Like this:
<div class="row blogitems">
{% for post in site.posts %}
{% assign mod3 = forloop.index | modulo: 3 %}
<div class="col-md-6"><p>{{ post.title }}</p></div>
{% if mod3 == 0 %}<div class="col-md-6"><p>THIS IS NOT A POST</p>{% endif %}
{% if mod3 == 0 or mod3 == 2 %}</div><div class="row blogitems">{% endif %}
{% endfor %}
</div>