You can only have OneCare on 3 PCs for the same license, meaning installed and activated with a single LiveID. You cannot purchase additional licenses for the same subscription.
However, you can install OneCare on the 4th PC and then activate with a different LiveID, creating a new second subscription, and providing credit card information or a retail product key to activate. This will allow OneCare to be installed to a total of 3 more PCs on this second subscription.
Since your OneCare subscription for the first 3 PCs was provided with your Qwest subscription, it should be automatically renewed after a year as long as you are still a Qwest subscriber. You'll need to contact Qwest about this, though.
-steve