random post thumbs

<?php //to show four random post thumbs
$temp = $post;
$wp_temp = $wp_query;
$thumb_posts = new WP_Query(array(
'meta_key' => '_thumbnail_id',
'post_type' => 'post',
'posts_per_page' => 4,
'orderby' => 'rand' ));
if($thumb_posts->have_posts()) : ?>
<div class="header-thumbs">
<?php while($thumb_posts->have_posts()): $thumb_posts->the_post();
echo get_the_post_thumbnail($post->ID,array(240,170));
endwhile; ?>
</div>
<?php endif;
wp_reset_query();
$wp_query = $wp_temp;
$post = $temp; ?>