Can you downgrade the firmware on v4.x and v2.x single bay My Cloud units? Yes. However, the problem lies with obtaining the older firmware. WD has blocked access to downloading earlier versions of their firmware (for what ever reason). See the following thread for more information on firmware versions and how to downgrade.
WD My Cloud v3.x, v4.x and v2.x Firmware Versions Download Links
http://community.wd.com/t/wd-my-cloud-v3-x-v4-x-and-v2-x-firmware-versions-download-links/148533
There are other discussions on how to downgrade that one can find using the forum search feature (magnifying glass icon top right).
Just be aware that downgrading firmware carries risks. When downgrading one is reintroducing potential security vulnerabilities that were patched by later firmware. And one may be reintroducing other problems that were fixed by the later firmware.
