Want make a template showing category

I want a page to show posts from a specific category and I’ve tried this:

I made a template just like the archive.php, and placed this before the loop,

<?php
query_posts('cat=4');
?>

the thing is that the page have subpages and has a menu in the sidebar based on this:

<?php
if($post->post_parent)
$children = wp_list_pages("title_li=&amp;child_of=".$post->post_parent."&amp;echo=0");
else
$children = wp_list_pages("title_li=&amp;child_of=".$post->ID."&amp;echo=0");
if ($children) { ?>
<ul>
<?php echo $children; ?>
</ul>
<?php } ?>

But, when I use anything but the default template (page.php) the sidebar menu of that page just disappear. Can’t I list subpages when the page uses another template other than the default?

Any ideas how to do this? Thanks!