usually the 4th socket is a fm/dab outlet depending on the type of communual system installed. have a look at the aerial and see if theres a bullhorn or round aerial on it further down the pole also a dab aerial which is T shaped. check the socket closeley and see what it says either fm/dab or return.
if you want a second sky box in the bedroom or somewhere then a second set of cables will nead to be ran from the multiswitch location to that point. Trouble is there may not be enough outlets on the switch to accomodate this and getting it replaced to cope could be expensive and the landlords are usually unwilling to do this in case other residents notice and want it also.
you have 2 options.
1. run a cable from the RF2 output ion the back of your sky box to your second location this will pass through sky AND freeview as long as the aerial is plugged into your sky box.. you will only be able to watch the same channel on sky as you see at the boxes location. if the socket in the lounge says return then plug RF 2 into this and this will be wired to one bedroom (or a distribution amp in a cupboard if feeding more than one room). On a direct connection you can fit a magic eye and remotely control you sky box in the second room.
2. wireless transmitters plug into the scart outlet of your sky box (vcr for example) and transmit to a reciever plugged into the scart input of the second tv. Im not a fan of these as i have seen many suffer with interference. you will only get the sky channel as well NO freeview.
Hope this helps mate any probs answer back!!!!