In software, backward compatibility is generally achievable, but forward compatibility can be quite problematic. Also, keep in mind that there is not only one version of the ojdbc7.jar. The SQL Developer 4.0 EA releases might be able to connect to a database using the version that ships with Oracle 12c (an unsupported configuration, of course), but earlier versions fail entirely.
Best to wait for the 4.0 production release. Then you'll see which ojdbc it ships with and how much latitude it provides to select an alternate version.
SQL Developer Team
Thanks, Gary. No worries.