Using php to add classes to looped data in WP e-commerce - php

Question

In WP e-commerce, I have the following in my products page template:
<ul id="product-categories">
<?php $counter = 1; ?>
<?php wpsc_start_category_query(array('category_group'=>get_option('wpsc_default_category'), 'show_thumbnails'=> get_option('show_category_thumbnails'))); ?>
<li class="four columns<?php if ($counter % 4 == 1){echo ' alpha';}else if ($counter % 4 == 0){echo ' omega';} ?>">
<?php $counter++ ; ?>
<a title="<?php wpsc_print_category_name(); ?>" href="<?php wpsc_print_category_url();?>">
<?php wpsc_print_category_image('auto', 'auto'); ?>
<span><?php wpsc_print_category_name(); ?></span>
</a>
</li>
<p><?php echo $counter ?></p>
<?php wpsc_end_category_query(); ?>
</ul>
As you can see, for every 4n-3th item, I am trying to add an alpha class, and to every 4nth item, I am trying to add an omega class to my list items, such that the display works with a grid framework.
My issue is, start_wpsc_category_query does not appear to be 'looping' like a standard WP loop, as my echoed counter value remains at 2 for every item, hence no class is added.
Does anyone know how to fix this code to make the counter value increase each time, or have a better (PHP) way of addressing the problem if you've dealt with WP e-commerce before?
Another way of approaching would be to write a foreach loop for WP e-commerce categories, if anyone knows how to do this.
Thanks a million in advance.