avoiding the IPv4 charge

Hello.

You need to create a snapshot from an existing instance and start it IPv6-only.
https://lightsail.aws.amazon.com/ls/docs/en_us/articles/amazon-lightsail-ipv6-only-plans

You can migrate an existing dual-stack instance to an IPv6-only plan. Before you begin, we recommend that you review the IPv6-only considerations section of this page.
To migrate, create a snapshot of your dual-stack instance, then create a new instance from the snapshot. Select the IPv6-only networking plan during the create instance workflow. For detailed information about this procedure, see Create a Lightsail instance from a snapshot.
To migrate from an IPv6-only instance plan to a dual-stack plan, select the Dual-stack plan instead.

Looking at this document, it seems that the prices are different for IPv4 and IPv6 Lightsail bundles.
https://aws.amazon.com/jp/blogs/compute/announcing-ipv6-instance-bundles-and-pricing-update-on-amazon-lightsail/

Hi,

You can find the updated prices for Lightsail instance bundles here: https://aws.amazon.com/blogs/compute/announcing-ipv6-instance-bundles-and-pricing-update-on-amazon-lightsail/

You cannot remove the IPv4 address from an existing instance, but you can create a new IPv6 instance from a snapshot of your current instance if the instance blueprint (application and OS) is supported on the IPv6 bundles.

Please refer to this link for details: https://lightsail.aws.amazon.com/ls/docs/en_us/articles/lightsail-how-to-create-instance-from-snapshot

We hope this answers your query. Please let us know if you have any further questions.

Thanks.