Answer by Michael for Can we count the Wordpress Loop

$wp_query->current_post is the build-in loop counter, starting with 0 zero for the first post in the loop.

so the line some code to be executed could translate to:

<?php if( $wp_query->current_post > 0 && $wp_query->current_post % 5 == 0 ) { ?> some code to be executed<?php } ?>