If you have OneCare installed on the 4th PC and activated under a different subscription you will need to reinstall OneCare and activate with the LiveID from your other susbcription.
If OneCare is in trial mode on the 4th PC and not activated, but the activation/subscribe dialog contains a different LiveID than the one for your subscription, see this link to reset the LiveID - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2776450&SiteID=2
If OneCare is not installed on the new PC, install it from http://onecare.live.com and subscribe/activate with the LiveID for your subscription. On any of the PCs in your subscription that is a Hub, select the details from the computer that you are removing in Manage your OneCare Circle and click Remove this PC.
-steve
