<?php function post_style()
{
    global $post_count;
    echo "tab".$post_count;
    if ($post_count % 4) {
        unset($post_count);
    }
} ?>
<?php if (have_posts()):
    while (have_posts()):
        the_post(); 
		$post_count++;
		?>
<div id="<?php post_style(); ?>"> içerik </div>
<?php endwhile; ?>