If one uses the forum search feature they’ll find this question about the difference between v3.x/v4.x versions and the newer v2.x versions has been asked before (as recently as last week) and discussed previously.
No at this time one CANNOT flash the v2.x firmware to the older v3.x or v4.x My Clouds.
The main difference between the older and newer versions is hardware and a different OS. See the following threads for more information:
https://community.wd.com/t/wd-my-cloud-2-x-firmware-what-else-has-changed/160303
https://community.wd.com/t/change-firmware-from-ver-4-to-ver-2-on-original-my-cloud/158542
