postfix force ipv4

 inet_protocols = ipv4 will not allow restart postfix.

Update: I confused protocols with interfaces

Se7p 21 10:24:30 server.foreveryours.us postfix[6753]: fatal: config variable inet_interfaces: host not found: ipv4
Sep 21 10:24:31 server.foreveryours.us systemd[1]: postfix.service: control process exited, code=exited status=1
Sep 21 10:24:31 server.foreveryours.us systemd[1]: Failed to start Postfix Mail Transport Agent.
-- Subject: Unit postfix.service has failed

Edit /etc/sysctl.conf and add the following at the end of the file.

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

After, run the command:

sysctl -p

Its really really really important to set

inet_protocols = ipv4

in postfix config file....../etc/postfix

Studio4host showed how to disable IPv6 absolutely, Postfix will use IPv4 only in this case.