This page does not track all issues or bugs but should do a good job highlighting many. The development team is working hard to develop one off patches (for some issues) as well as a 4.0.1 re-release. As of the time of this post we have 2 one off patches available via metalink.
We have also updated our Oracle APEX OTN page to link to this known issues page, in the "New in release 4.0" section. If you do encounter bugs with Oracle APEX 4.0 and wish to bring these to our attention please do use the Oracle APEX discussion forum or log bugs with Oracle support.