Just some feedback since I upgraded to 4.0 myself and I was not able to access the admin page after the upgrade. I disabled all my plugins by renaming the name of all my plugins in the file manager.
I enabled the plugins one by one by renaming the folder back to it’s original name and I found that the error was caused by the woocommerce plugin.
I’m running WP 4.0, Alora Theme and Woocommerce 2.2.4 (Also had the same issue with 2.2.3)
Hope this will help.