I installed and activated Wordfence on two different installs. One is running WP 4.0 and the other is WP 3.9.2.
When I login using the install with 4.0, it goes to a blank page at the URL http://www.MyWebSite.com/wp-login.php and just sits there.
I can click in the address box and hit enter, the Dashboard comes right up.
I am using Firefox 32.0.1 and Safari 7.0.6 on a Mac and get the same results.
If I deactivate Wordfence, the issue goes away.
Any suggestions?