Ads between posts in homepage

i need to add an ad between the 3d and 4th post experts in my home page

<?php while (have_posts()) : the_post(); ?>
					<!-- Start: Post -->
					<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
						<h2><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h2>
						<p class="postmeta">Posted on <a href="<?php the_permalink() ?>"><?php the_time('F jS, Y') ?></a> by <?php the_author() ?></p>
						<div class="entryContent">
						    <?php the_post_thumbnail(); ?>
							<?php the_excerpt(); ?>
						</div>
						<p>Posted in <?php the_category(', ') ?> | <?php edit_post_link('Edit', '', ' | '); ?> <?php comments_popup_link('No Comments', '1 Comment', '% Comments'); ?> </p>
					</div>
					<div class="clear"></div>
					<!-- End: Post -->
				<?php endwhile; ?>

is there a way to do this