So this is a notebook from work and would like to use it in such a way that it somehow has no relation to my data on my home pc with which I also surf the same fritz box… Or do I not have to worry because the notebook is not would somehow be able to record my internet consumption over the router?
Unless you have given approval and the notebook is not accidentally infected with a virus that sneaks into other devices and calls home nothing should happen.
Your worries are most likely unfounded. But you could configure LAN 4 as a guest port with some Fritz boxes, then there's no longer any connection.
The notebook can't record your data traffic.
If you want to play it safe then configure the guest access of the Fritzbox and connect the notebook there. Then the two networks are completely separated.
As a rule, a VPN client should be installed on the work laptop with which you can log into the company network. The connection is encrypted, so you don't need to change anything in the Fritzbox. Anything else would be a disaster in terms of data protection.
