IPv6 cannot directly talk to IPv4. Usually you would have dual-stacked clients that can access IPv4-only applications as well as IPv6-only applications. This allows to slowly shift more and more traffic from IPv4 to IPv6 by setting up more and more IPv6-only applications.
If you need IPv6 to talk to IPv4 directly, you need translation. Currently there is a specification called NAT64/DNS64 which allows access to IPv4 applications by IPv6 clients.
Hope this helps