I would like you to try a few things if you’re willing. My hope is to get that upgrade to your Z4.
As @rolandh and @southpaw have stated the phone model you have is the same as you would get from the Moto store. With this in mind, let’s see if we can make the phone update.
First let’s try something simple:
Turn off the phone, remove the SIM, turn on the phone and try the update again.
If that fails and you don’t mind doing a factory reset… that would be my next step.
After you made sure you got your phone backed up (we can talk about that too if needed), I would remove the SIM, reset, move through the setup and try the update again.
The last thing I can think of to try is to flash the OS using the Lenovo Rescue and Smart Assistant. I’ll hold off on those instructions till you maybe try the prior suggestions.