Thanks for the help! This solution worked for me too. I also manually updated and got the ‘Fatal error….pluggable.php’
I only had to replace my wp-settings.php file with the one George gave to fixed it. No other steps were necessary: https://github.com/WordPress/WordPress/blob/4.0/wp-settings.php
Just wanted to note for anyone else trying to fix this problem that the zip download of WP 4.0 did not include a wp-settings.php file. The update for 3.9 does have a settings file and line 122 does have include session.php like it is supposed to. I’m not sure why replacing this file worked, unless I failed to upload wp-settings.php file during the last update, which is possible.