Lost, Found, Can't : Stacks 3 Retrieval Problem?

After buying a new iMac, I lost all my RW addons including Stacks. I’ve downloaded the Stacks demo and entered my serial no for Stacks 3, but it’s insisting that I enter my serial No. for Stacks 4 Upgrade (which I haven’t bought), also. Have RW8.3
A) Can I just retrieve my Stacks 3? If so how?
B) If I decide to buy the upgrade, can I simply buy the Stacks 4 upgrade and enter the serial number alongside my Stacks 3 number?

You can download an earlier version of stacks (Stacks 3.6.9 was the last version from the archive here:

I’d probably get the stacks 3 installed with the serial number first and then use the upgrade option.

It sounds like you have the S3 serial number, but if you have problems, you can find info on looking it up here: