选择第三个元素，并从那里选择wordpress循环中的每个第四个元素

I need to slect every third element and from there on every fourth in a php loop for wordpress custom posts. So i have to select 3,7,11,15....

I actually select every third element but thats not the thing i need for the function i need:

<?php $counter = 1; ?>
<?php if ( $loop->have_posts() ) : while ( $loop->have_posts() ) : $loop->the_post(); ?>

<?php if($counter%3 == 0 : ?>
     <p>Do this on the third and from there on on every fourth element</p>
<?php else : ?>
     <p>Do something else.</p>
<?php endif; ?>
<?php $counter++; ?>

<?php endwhile; ?>
<?php else: ?>
<?php endif; ?>

Use this instead:

if( $counter%4 == 3) :