If you are using a default theme and not a customized one, can you try to use for example theme 2013?
Also try solutions on this post, seems like this update is causing many problems.
https://wordpress.org/support/topic/cant-login-after-400-update?replies=33
I just updated my WordPress to 4.0 two days ago and everything is okay.
Have you tried clearing the cache of your browser or using another browser for this?
