add class to the 1st, and every 3rd and 4th element

field.tpl.php

<?php $max_delta = count($items) - 1; ?>
<div class="<?php print $classes; ?>"<?php print $attributes; ?>>
  <?php if (!$label_hidden): ?>
    <div class="field-label"<?php print $title_attributes; ?>><?php print $label ?>:&nbsp;</div>
  <?php endif; ?>
  <div class="field-items content"<?php print $content_attributes; ?>>
    <?php foreach ($items as $delta => $item): ?>
      <?php
        $classes = 'grid-3 ';
        $tmp = $delta % 4;
        // Applies to the first item, then every 4th item
        if ($tmp == 0) {
          $classes .= ' alpha';
        }
        // Apples to every 3rd item
        elseif ($tmp == 3) {
          $classes .= ' omega';
        }
      ?>
      <div class=" <?php print $classes; ?>"<?php print $item_attributes[$delta]; ?>><?php print render($item); ?></div>
    <?php endforeach; ?>
  </div>
</div>