The first scenario is only possible trough the API. As for the second scenario, if one signer declines, then the package will be put back in your drafts folder. To complete the package, you can either delete the signer who declined if this signer doesn’t have any signatures on documents that signing has already begun or update the signer to an email address you have access to and resend the package to complete it.
for the second scenario, we essentially want to avoid asking the signers to sign again if they already have signed.
With the option you are proposing, would we have to download the document and create a new transaction with only the 4th signer on it or we will have to delete all the signers who have already signed and then send the package out to the signer yet to sign?