May be there in no hook for that or i don't know about that, But You can do something like this:-
<?php
$postnum = 0; // Set counter to 0 outside the loop
if (have_posts()) :
while (have_posts()) : the_post();
// Do regular stuff
$postnum++; // Increment counter
if ($postnum % 5 == 0){ // If the remainder of the counter value divided by 5 equals zero
// Do special stuff
}
endwhile;
endif;
?>
Hope this will help you.