> The IPv4 address space should have been included in the IPv6 space […]

See IPv4-mapped ("IPv4-compatible") IPv6 addresses from RFC 1884 § 2.4.4 (from 1995) and follow-on RFCs:

* https://datatracker.ietf.org/doc/html/rfc1884

* https://en.wikipedia.org/wiki/IPv6#IPv4-mapped_IPv6_addresse...