The af-to in #OpenBSD #PF is so flexible that it allows you to host IPv4 services within a #IPv6 only network, only requiring dual-stack on the firewall/router. The 'pass' rule even allows for selective port decisions on where the IPv4 address might be used for other IPv4 only services #BSD:

pass in on $ext_if inet proto tcp to (egress:0) port 12345 af-to inet6 from 2001:db8:dead:beef::1 to 2001:db8:dead:beef::/96 port ssh