alt Hacker News> The Version field must remain 4.
And at the same time the address format and IP header is extended, effectively still splitting one network into two (one of which is a superset of the others)?
A fundamentally breaking change remains a breaking change, whether you have the guts to bump your version number or not.
The central idea is that a IPv4x packet is still a globally routable IPv4 packet. The extra stuff all goes in the body of the packet.