how to retrieve deleted messages 4 months ago

I should have added, but as it's new I forgot about it - in iOS 16 you can restore deleted messages from a Recently Deleted store, which holds them "within the last 30 to 40 days" according to Apple's support page Delete and recover messages on your iPhone or iPad

However, as your messages were deleted well beyond 40 days ago they cannot be retrieved.

If you have iCloud messages active it's gone for good.

If you don't have iCloud messages active and backup your iPhone or other iOS device to a computer you could use that backup to restore the device, and if the message is in the backup it will be restored with your other data.