If you've got cable to spare, run a cable from the 4th socket into the loft or wherever, leave plenty of spare cable coiled up and then put the terminator on the end (you'll need an F-type plug and a coupler on the end of the cable of course). That way you've got the 4th feed ready if you need it in future. If not, then if the terminator goes duff (and they do sometimes) it's a damn sight easier to change....