Since releasing APEX 4.0 we have identified a number of issues.
Many are directly related to upgrading applications from previous releases.
The known issues are outlined on OTN here: http://www.oracle.com/technology/products/database/application_express/html/4.0_known_issues.html
As with any upgrade we strongly recommend you test your applications in a development environment before applying to production.
Our development team is working very hard on these issues.
Many issues have workarounds provided.
Where appropriate we are also releasing patch set exceptions which can be downloaded from Oracle Support and installed in your environment.
We intend to roll all of these fixes into an APEX 4.0.1 release.
Before you ask - I can't provide dates for APEX 4.0.1 release!