In Short Yes - Depending on what else is downstream (Load) of the spur.
4M is fine as long as the spur is not already part of another spur (Longer) - (Because no spur should be longer that the circuit)
Make Sense? Call a qualified Electrician.
Mark - Fuse Electrical Services
You can have as many sockets as you want after that fused spur. The maximum current that can run through them sockets is limited to 13 Amps (the rating of the fuse inside the fused spur).
My advice is to put that socket within the ring and not after the fused spur unless you want to use that fused spur to isolate a built in, wired through a connection outlet (or difficult to unplug) appliance for maintenance purposes in which case that socket will be connected on the load side of the fused spur.
Hi yes you can as long as the total load is not likely to go over the 13 amps, I'm assuming there is already a load on this fused spur.
I must stress as always this should be done by a registered electrician, who will assess the suitably before carrying out the work.