pagination with wp_query

on this page i’m using the following code to pull in the 25 most recent posts from category 4.

<?php
$custom_query = new WP_Query( 'posts_per_page=25&cat=4' );
if ( $custom_query->have_posts() ) : while ( $custom_query->have_posts() ) : $custom_query->the_post(); ?>
<h3><?php the_title(); ?></h3>
<?php the_content(); ?>

<hr class="homenews" />

<?php endwhile; else : ?>
<h3>Sorry...</h3>
No posts were found.

<?php next_post(); ?>

<?php endif; ?>

but how do i add a link at the bottom of the page that says “click here for earlier posts”, and leads to a page that lists the 25 items posted before that???

please advise. thanks in advance.