I was having all sort of problems after upgrading to version 4. I am not comfortable with all the SSH editing but I got help from the WD support team to downgrade with minimal SSH work. It's pretty easy.
Here are the instructions:
Download the following firmware version and unzip the .deb file (File sq-030401-230-20140415.deb.zip) http://download.wdc.com/nas/sq-030401-230-20140415.deb.zip
Then download Winscp and install it: http://winscp.net/eng/download.php
You need to "turn on SSH" from Settings > Network in Wd My Cloud web interface.
Now run Winscp program you have installed earlier. In the Login window
select "SFTP" in File protocol dropdown
Enter your My Cloud IP adress in the host name
The default usename is root and password is welc0me (its the number 0 in welcome and not letter O)
Once you login you will see 2 panes in the window.
Go to the right pane and naviagte to the folder "etc".
Once in the "etc" folder scroll down to the bottom and you will see a file called "version".
Double click to ope the file and change the version name to 03.04.01-219 and save the file.
Thats all the SSH work. Now close and exit Winscp application.
Reboot My Cloud. (Settings > Utilities > Device Maintenance > Reboot)
After it has rebooted go into settings > firmware and click on "select from file". Naigate to the sq-030401-230-20140415..deb file you have downloaded earlier and update.
DONE. Now you will be running the previous version. Bye Bye buggy frustating version 4.
MKAE SURE TO TURN OFF AUTO UPDATE FIRMWARE.
PS: It worked for me so hopfully it will work for others also. I am a noob, can some experts check it and provide feedback.
Just to let you know - try it at your own risk. I can't be held responsible if something goes wrong during the process.
[Edit - Bill_S]
Note: Users, please be aware that making changes to your drive through SSH can void your warranty.