Custom 404 Pages

I'm trying to customize the 404 page (404.php). I want to keep the primary navigation at top, but remove the header just below that.

<?php
get_header(); ?>

appears to be the line that loads both the primary nav and the header.

How do I include only the primary navigation? 404.php can't be edited in the page editor, hence I cannot turn off the header there via Disable Elements.

Thanks in advance!