Thank you for the reply ! I had thought first that I should use two wireguard interfaces on Raspi, one to connect from Win8.1, one to access to Fb4020, but then you said I needed just one interface, then the wg-subnet on FB4020 and Raspi have to be the same.....
On the other hand, it will be deleted anyway, once Fb4020 gets its own Internet connection, then it's not going to sit behind raspi anymore;;
In any case, I will wait till the second internet comes in terms of wg.
Thanks a lot for all your tutorials!