Adding a class to post_class

Hey everyone,

I want to add a class of span4 to my article posts but there is a <?php post_class(); ?> there and I dont know how to add my own class of span4 to it.

My code for the article section is:
<article id="post-<?php the_ID(); ?>" <?php post_class(); ?> class="span4">

I didn’t think the above would work but gave it ago. Does anyone know how to add span4 just strictly to my article posts?

Thanks!
Megan