That behavior hasn't bothered me in quite some time. Probably the definitive answer (or at least the most comprehensive theory) comes from John McGinnis in this thread: SQL Developer: Not able to view tables
I am under the impression the 'Out of Band Breaks' issue shouldn't occur with the 4.0 EA's since those are hardwired to use a more current version of the JDBC driver, but perhaps something about your environment still makes you susceptible. You should always mention your OS, database, JDK versions.
If all else fails, have you tried doing a clean install? That is,
1. Unzip the product into an empty directory
2. Only start it up after either...
2a. Removing/renaming the directory structure containing your user settings, or
2b. Pointing to some other directory structure via an AddVMOption -Dide.user.dir=<writable_path> line in your sqldeveloper.conf file
3. Start it up without migrating user settings.
Hope this helps,
SQL Developer Team