The two versions of the single bay My Cloud use slightly different hardware. Currently (and officially) one cannot install the v2.x firmware to the older generation My Clouds that run v4.x firmware. Same goes in reverse, cannot run v4.x firmware on the newer generation v2.x firmware.
Gen2 (v2.x):
Processor: Marvell armada 370 (2core, 800MHz)
Memory: 512 MB Ram
Gen1 (v3.x/v4.x):
Processor: Mindspeed Comcerto C2000 (2 core, 650MHz)
Memory: 256 MB Ram
The Gen 2 uses a form of Busy Box as the OS while Gen 1 uses a cut down version of Debian.
What one can do, unofficially, if they have a v4.x My Cloud is install alternate firmware to the My Cloud. See the following threads.
https://community.wd.com/t/clean-os-debian-openmediavault-and-other-firmwares/93714
https://community.wd.com/t/synology-os-on-wd-my-cloud-new-hope/95650
