Doing a search of this subforum (via the search icon) yields several threads that discuss how to roll back the Firmware on the v4.x My Cloud devices.
http://community.wd.com/t/wd-my-cloud-v4-x-and-v2-x-firmware-versions-download-links/148533
http://community.wd.com/t/easy-method-to-downgrade-to-previous-version-of-the-firmware/93489
