Login problem 404

Redirects are normally handled by a config file like .htaccess. You could try renaming that file to disable any redirects to see if that helps you get in.

Of course, finding the cause of the error is key. Did you add or update any plugins, update WordPress, or make any changes prior to the error occurrring? Does anything different occur when you rename the plugins folder? Are you using any plugins that manipulate the admin URL for security?