You'd think it would be long enough for people to realize that v6 is backwards compatible! Yet no, here we are, constantly dealing with people making the same damn claim that it isn't every single time a v6 story is posted.
v6 is about as backwards compatible with v4 as it's possible to be. If you have a way to make it more backwards compatible then I'd love to hear it, but when I ask this all I ever get are things that don't work, or things that v6 already does.
No, it's not. If I have an ipv6 network, an ipv4 address is invalid. It's that simple.