Problem on WDShareSpace

this afternoon my WD ShareSpace 4 x 1TB drives no longer responding.
The disks 1 and 2 are in RAID1, the disks 3 and 4 are in extend.
No answer on the web, does not respond via cifs. The disks and network however recorded activity (according to the frontal LEDs)
I tried to restart it but the close button not working even if held longer.
I finally removed the power cable.
I rebooted, and finally after a long time the system is rebooted even if slowly.
The web interface is slow.
Sharing windows works … but the files can not copy them.
I open the status of hard disks and I understand that 1,2,3 are “good” and the disc 4 is in a state “new” and there is a small icon next to the “Safely Remove Disk”.
I do not know how to proceed. I do not by error messages on the disk, so do not know if he has mounted well or what.
What is the best way to proceed?
Thank you

It would be best to contact WD Support about this. You can do so over the phone or via E-mail.