A custom template is needed. The main.cf config file works like last occurance counts so it’s possible to just add a custom fragment at the end instead of overwriting:
mkdir -p /etc/e-smith/templates-custom/etc/postfix/main.cf
echo 'inet_protocols = ipv4' > /etc/e-smith/templates-custom/etc/postfix/main.cf/90ipv4
signal-event nethserver-mail-server-update