Hello, how can I backed up WD EX4 to dropbox??? 
If you mean backing-up your entire WD My Cloud EX4 to your Dropbox account then it has to be done manually or by means of an app able to upload files for you.
If you mean syncing your Dropbox account with your NAS then all you need to do is install the Dropbox app from your NAS’s dashboard, link your account, and select the share that will sync with your account. Instructions related to installing the app are available in your NAS’s user manual, page 84 (89).
