Transferring my data to a "new" 4s

Oopsssss, that a big problem.

After iOS 11, if you can't remember that password, but you still have your old iphone, you can make a new encrypted backup, using a different password (see this guide, at section "If you can’t remember the password for your encrypted backup"):---> About encrypted backups in iTunes - Apple Support

But, unfortunately, you can't do it using an older iOS version.

That's a difficult situation... can you create a new full backup using iCloud and try to restore data from there?

Giulio

RoGos,

I would investigate if the previous iP4S owner, before giving you that iphone, has performed ALL the steps provided in this support guide: ---> What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Regards

Giulio

It might be asking for the backup password. If Encrypt Backup is checked in iTunes then you will need the password you entered when you created the first encrypted backup of the iPhone 4. That might have been years ago; the backup password never changes unless you explicitly change it.

Are you absolutely sure about point #3?

Regards

Giulio