Please no, custom templates from copies of the original templates should be used only to override a default value or option.
In this case, simply add a new file called 91ipv4dns and put that single line in it (dns_v4_first on).
So, to recap, the best practice is to avoid copying standard templates to templates-custom directory unless you need to override a default behaviour.
And the commands for @lldm30 are:
mkdir -p /etc/e-smith/templates-custom/etc/squid
echo "dns_v4_first on" >/etc/e-smith/templates-custom/etc/squid/91ipv4dns
signal-event nethserver-squid-save
@mrmarkuz I think that we may need to expand our documentation to add some best practices.