With all the complaints recorded in this forum I feel WD should remove the FW4.00 to prevent further damage. Meantime they should do 2 things:
Help those who have upgraded to FW4.00 to downgrade back to FW3.00 which worked well for them.
Rework and retest FW4.00 and release it only when it is proven to be stable and robust.
WD should work hard to contain the damage and do everything possible to help users regain confidence in the WDMyCloud.