There are no API changes from RC4, just bug fixes and doc fixes. If you're upgrading from something earlier than RC4, you will definitely need to review the list of changes here:
If you can, please upgrade to this RC and let us know any issues you encounter! A future upgrade from RC5 to 1.0-final will then be trivial for you, as at the worst we may just need a few more small fixes -- no big API changes.