It would help if you provided some additional information so people would know what you are talking about.
What app are you using on the phone that is generating that error number?
What is your phone OS?
Are you connecting to the My Cloud through the local network or through your cellular data connection (ie remotely)?
What is the exact text of the full error message?
There are a number of prior discussions of a 904 error message. You may want to review those threads to see if they apply or if there was a solution.
http://community.wd.com/t/wd-my-cloud-setup-problems-access-to-device-is-horribly-slow/145495
http://community.wd.com/t/cant-get-remote-access-to-work-anymore/94738
http://community.wd.com/t/wd-sync-unable-to-connect-to-the-remote-server/135289/3
http://community.wd.com/t/need-help-new-mycloud-and-frustrated/95172
