nth_child_4n_field_template.module

<?php

function nth_child_4n_ds_field_theme_functions_info() {
  return array('theme_ds_field_nth_child_4' => t('Highlight 4th child'));
}

function theme_ds_field_nth_child_4($variables) {
  $output = '';

  // Render the items.
  $output .= '<div>';
  $i = 1;
  $max = count($variables['items']);
  foreach ($variables['items'] as $delta => $item) {
    $classes = $i % 4 ? 'nth-child-4n' : '';
    $output .= '<div class="' . $classes . '"' . $variables['item_attributes'][$delta] . '>' . drupal_render($item) . '</div>';
    $i++;
  }
  $output .= '</div>';

  return $output;
}