Most v4 support is through a gateway. You can't tell the user's IP address from the wrong side of the gateway, for example - only the gateway's address. The user isn't on v4, the gateway is.
Yeah, so you still reach the user, it's just probably less efficient than the all-v6 route.
Edit: Oh, you mean if they want unsolicited inbound traffic? Sure, but that's only a thing for services. I mean you can have a default-allow firewall to home devices but really shouldn't.