You can use a wired TV-Link between rooms, it connects to the RF2 output of the Sky box. You can only watch the same Sky channel in each room though, or a terrestrial one.
http://www.letsautomate.com/10570.cfm
Otherwise its another Sky box which would need a separate downlead from your LNB, but I guess you are only using three at the moment so you have one spare.