How to insert an element after the X and Y element in a list?

Question

PHP

There is a simple list

<?php foreach ($this->rows as $k=>$blah) : ?>

<div class="someclass">blah-blah</div>

<?php endforeach; ?>

How to insert something after every second element and after every fourth, so that the output is like this?

<div class="someclass">blah-blah</div>
<div class="someclass">blah-blah</div>
<div class="someclass">FOO</div>
<div class="someclass">blah-blah</div>
<div class="someclass">blah-blah</div>
<div class="someclass">BAR</div>
<div class="someclass">blah-blah</div>
<div class="someclass">blah-blah</div>
<div class="someclass">FOO</div>
<div class="someclass">blah-blah</div>
<div class="someclass">blah-blah</div>
<div class="someclass">BAR</div>

и так далее