The server only goes into RAID Initialization or RAID verification mode when the server experiences an unclean shutdown or inadvertent power loss.
The first time (and only first time) the server experiences the unclean shutdown, it will go into RAID initialization mode.
If the server experiences another unclean shutdown thereafter, it will go into RAID verification mode. RAID verification is somewhat faster than RAID initialization, depending on amount of data stored on your server.
It is highly recommended that with any type of NAS/server with RAID, you use a UPS to protect your systems from sudden power loss and WD Sentinel DX4000 is no exception to that.
Here's a KB that's posted on WD Support Knowledge base regarding the supported UPSs: Answer ID 9403