Getting a replace drive, will be able to revert to Fw 3.x?

Im doing a RMA for my current drive. most likey it will come with a 4.x FW.

Would i be able to flash older 3.X FW?

You can downgrade yes. Different methods are available from this forum.

http://community.wd.com/t5/WD-My-Cloud/Tutorial-SSH-Method-Downgrade-v4-firmware-to-v3/m-p/812311/highlight/true#M23232