Try accessing the My Cloud Dashboard (http://wdmycloud or http://wdmycloud.local) with a web browser and perform a System Only reset via the Settings > Utilities page.
Baffling as it may seem to some there are any number of reasons to downgrade the size. Maybe the 4TB drive died and the 2TB is all one has free. Or one could be wanting to use a 4TB drive elsewhere. Like in a newer better NAS or in their PC.
Thanks for the suggestion I have already tried but it doesn’t work, from my modem / router I tried to find the connection, to start it via ip, but as if it didn’t connect to the network