I believe so. I have instances on 4.1 and 4.2 and they are all configured to use SMTP credentials for sending mails using APEX_MAIL. But before you upgrade your current instance, I strongly recommend creating a separate instance for testing your 4.0 application in the new environment. I would do that with VMs that I could quite easily blow away.
Thanks for your well wishes. Happy Holidays and have a great new year ahead!