I guess you can have clients that purely talk IPv6 talk with destinations (edge locations) that only talk IPv4, without requiring the whole end-to-end connection and intermediate routers to talk IPv4? You'll just have a router at the end which translates this IPv4-in-IPv6 packet into a pure IPv4 packet and everything will work.
For a v6-only host to talk to a v4-only host you need a router performing stateful NAT64 at some point (it's fundamentally impossible to statelessly route IPv4 packets to v6 hosts because there aren't enough v4 addresses). While in theory that router could live anywhere on the IPv6 network (and packets are routed to it via normal IPv6), normally you'd have it on the 6-only host's network (and coordinate with their DNS servers if they wanted to be able to connect to 4-only hosts by hostname via DNS64).
It's a little ugly, but it's pretty much the same situation those v6-only hosts would be in if they were v4 hosts without public addresses (behind NAT), and they get to use ordinary direct connections when talking to a v6-enabled host on the outside.
