You can't selectively use IPv6 in the WAN and IPv4 in the LAN.

How would an IPv4 client send a packet to an IPv6 server? The IPv4 packet has only 4 bytes to spare for the destination address.

You can't.

Anything longer, even if it's just 1 bit, would require a new IP version.

You can run a 4to6 NAT, where specific v6 addresses (the ones you’re talking to) are aliased to reserved v4 addresses out of the PUA.

	▲	orangeboats 4 hours ago \| parent [-]
		I have seen people suggesting this before, but it's problematic for a few reasons: - If you are managing the 4to6 mappings manually, it's obviously not going to be scalable. Just to give people an idea, websites nowadays can connect to 10+ IPs in a single page due to CDN load balancing and stuff. - If you are handling the mappings automatically, such as by monitoring DNS queries and creating dynamic mappings based on that, the issues become more insidious: applications with hardcoded IP literals would fail without an obvious reason.