Does IPv4 and IPv6 talk to each other?

Definitely. IPv4 has no techniques for talking to IPv6. They are entirely different network stacks, although there are a lot of similarities. IPv6 does have a representation for native IPv4 addresses so they can be routed appropriately. Essentially, the IPv6 country would have limited access from other countries using IPv4. Gateways could enable access to email, web sites, and other services, but this would need to be handled on a roughly case by case basis.

The providers of cable modems and other such equipment are scrambling to be able to provide IPv6 capability. In part this is because they will need to provide dual stacks on hardware with limited memory capacity.

Be prepared to relearn border security, as IPv6 does not use NAT. Different techniques will be needed to secure networks from the external access.

If a country switched completely to IPv6 and still wants to access some old IPv4 datacenters or Networks ressources, it needs the service of NAT64 boxes which can still translate IPv6 to IPv4.

A bit more details about all the transitions to IPv6 tools: http://www.fastlaneus.com/blog/2011/08/19/transition-to-ipv6-at-the-service-providers-nat64-6rd-ds-lite-6pe-6vpe-and-carrier-grade-ipv6-cgv6/

Fred