Hello flannnn–
I tired that and it defaults to something else, either in the “loop” or somewhere else. I tried removing 404.php and the code below from the loop.php but them I get a blank page (if I remember correctly):
<?php /* If there are no posts to display, such as an empty archive page */ ?>
<?php if ( ! have_posts() ) : ?>
<div id="post-0" class="post error404 not-found">
<h1 class="entry-title"><?php _e( 'Not Found', 'twentyten' ); ?></h1>
<div class="entry-content">
<p><?php _e( 'Apologies, but no results were found for the requested archive. Perhaps searching will help find a related post.', 'twentyten' ); ?></p>
<?php get_search_form(); ?>
</div><!-- .entry-content -->
</div><!-- #post-0 -->
<?php endif; ?>
Thanks for trying.