Can I install 2 3TB disk (alrready I have) in a WD My Cloud EX4 Diskless, or it only support 4TB disks?
You can certainly install your existing 3 TB disks…but if setting up RAID, they must be identical model from same vendor.
+1 yes for sure like Cybernut1 says
In the new firmware 1.04.04 you can use drives of 5 tb and 6 tb too 
