From the sound of it, it would be beneficial if you did. If the current wiring is safe, then it's not absolutely necessary; it depends on your situation. I would say it needs rewiring as soon as practicable, rather than immediately because from the sound if it, it doesn't pose an immediate risk. If now is a good time then do it now, if it's not then start planning when you are going to do it. Be absolutely sure that all your fuses are rated at less than what your cables are rated at. You may find that the 4mm cable is on a 30A fuse and if those sockets are wired in 2.5mm cables from the JB, then that is too large. If it is 30A (or more) then either rewire it now or down rate the fuse until you can.