Disable IPv6 on Postfix

I would like to disable IPv6 permanently from postfix.
when I set the line inet_protocols = ipv4 in the main.cf it gets overwritten. so how can I change this permanetly?

Thank you for your Help

With a custom template:

thank you for your fast replay.
I’ll get the error Message:
Can’t open directory /etc/e-smith/events/nethserver-mail2-server-update

the correct signal-event was ignal-event nethserver-mail-server-update

now I have the line twice:
in the beginning it says
inet_protocols = all
and the last line ist
inet_protocols = ipv4
so the last one overwrites the one prior?

IIRC it does, if not you need to overwrite the fragment with a custom one. You may check with

postconf | grep inet