First try performing a 40 second reset via the reset button on the back of the My Cloud. Sometimes that fixes issues that occur after upgrading the firmware.
Next, if you previously unbricked the single bay My Cloud, sometimes users (including myself) had problems with the Dashboard being unreachable after upgrading the firmware from v3.x to v4.x. In my case I detail what worked for me to fix this issue in the following post/thread:
https://community.wd.com/t/unbrick-then-firmware-upgrade-equals-no-dashboard/136666/6
