I want to use Magento Connect to install a plugin. I am receiving a 404 page when I try to open it.
Maybe your admin/developers did their job and either removed or protected the connect manager.
1.) a potential security risk .
2.)And untested extensions might break your site.
That's why you should not use the connect manager directly on a production site.
Instead download the extension to a development site, test it, then move the code to production. To download it without the connect manager, you can use this tool: http://freegento.com/ddl-magento-extension.php
in the worst case, if you want immediate solution , Alternatively you can install the extension through FTP (like FileZilla). You can paste the extension key in this unofficial link and you can download the extension files. Let me know if you find any problems in this.
Note : As this is unofficial 3rd party site, there is no guarantee about security. but i tested many extensions, i did't faced any security problems....