On Amazon EC2, even with a properly configured HTACCESS file, permalinks will not work, resulting in 404 error pages. While this is documented, I found it hard to find:
Look for the heading “To allow WordPress to use permalinks”. In your Apache configuration file, within the section that starts with
<Directory "/var/www/html">, change
AllowOverride None to
AllowOverride All. If your Apache configuration file does not include this section, add it. Restart Apache and permalinks should work.