Another way could be to form an ipv4 subnet (or more?) which cost a little bit more than the others.
People can split their canister and put ipv4 site related logic to there and access the data with cross subnet calls. The capacity and performance of the subnet can be low, because canisters would be idle most of the time, aka wait either for the request or the response.
Over time, as the transition to ipv6 continues, the demand for ipv4 subnet would decrease.
Of course cross subnet calls are slower and more expensive, but this further incentivise the use of ipv6 sites.