Paging is just showing the repeat of first page posts

Hi can anyone help?

I’ve set up category-5.php to show all posts assigned to it and none of category-4.php posts.

The trouble is that the paging doesn’t seem to be working – not sure if that’s related or not – but it’s only showing the same set of posts (that are in category 4).

Any ideas what I’m doing wrong?

Thank you 🙂

<?php get_header(); ?>

<div class="clearBoth"></div>
<div class="container clearFix">
<div class="articles">
  <?php
$args = array( 'category_name' => 'Project Portfolio' );
$lastposts = get_posts( $args );
foreach($lastposts as $post) : setup_postdata($post); ?>
  <div class="grid_5">
    <a href="<?php the_permalink(); ?>">
    <?php the_post_thumbnail('portfolio-category'); ?>
    </a>
  </div>
  <div class="grid_7">
    <h3><?php the_title(); ?></h3>
    <p><?php the_excerpt(0,200); ?></p>
    <p><a href="<?php the_permalink(); ?>">Read more</a></p>
  </div>
  <div class="clearBoth"></div>
  <hr />
  <?php endforeach; ?>
</div>
<div class="paging">
  <?php posts_nav_link('&nbsp;','<','>'); ?>
</div>
<?php get_footer(); ?>