What are you trying to achieve by adding this extension ?
It sounds to me as if the cable you are trying to connect is one of those cheapo 'flat' ones, if so, they are no good to man nor beast.
It doesn't matter too much, as long as the same colours are terminated on the same terminations either end. It sounds as though the incoming cable is a drop wire, the orange will be the A leg, the white the B leg. Green, earth, possibly, if it's as old as you say.
The internal cable sounds correct also.
You *should* just need one pair in your new cable to connect to the orange wires, and one to to blue and white terminations.
What type of GPO box is it ?