The one part of your question that Chuck didn't answer is that you are limited to *3* computers in a Circle. So, at this time, the 4th PC would not "legally" be allowed to be part of the Circle and use the printer and Centralized backup via OneCare.
However, you can still point backup of the 4th PC to the same Share location used by the other 3 and you can still print from the 4th PC using Windows File and Printer Sharing - it just won't happen via the functionality in OneCare.
Note that you can add a 4th PC to your subscription and therefore your Circle, but it is against the terms of the license and there is not facility to add a second subscription to your Subscription LiveID.
-steve