The base_facebook class is distributed by Facebook, and it's their API class for normal communication between your site and them. In previous releases, we've renamed this class to prevent the type of conflict you're running into. We're investigating a few minor issues that will probably necessitate a 4.0.2 release, and if so, we'll get this change in there as well and out probably late next week.
If it's urgent for you to upgrade to 4.0 for some feature that's included, let us know, and we can likely provide some code changes for you. If not, I'd recommend just waiting a week. Another concern is what the Gallery Pro extension is doing with Facebook, as multiple extensions communicating with Facebook can cause other issues.
Hope this helps explain, and thanks for the bug report,
Alex
Upgrade to JFBConnect 4.0 crashes other component
