RFC4361 intends to set the same IAID/DUID for both DHCPv4 and DHCPv6. Previously, we didn't have a mode for that.
Of course, you could always set "ipv4.dhcp-client-id" and "ipv6.dhcp-duid" to (the same) hex string, but there was no automatic mode. Instead we had:
"ipv4.dhcp-client-id=duid" which sets the client ID to a stable, generated DUID. However, there was no option so that the same DUID/IAID would be automatically used for DHCPv6.
there are various special values for "ipv6.dhcp-duid" which generate a stable DUIDs. However, those values did not work for "ipv4.dhcp-client-id".
Solve that by adding "ipv4.dhcp-client-id=ipv6-duid" which indicates to use the DUID from DHCPv6's "ipv6.dhcp-duid" setting. As IAID it will prefer "ipv4.dhcp-iaid" (if set), but fallback to "ipv6.dhcp-iaid".