I would bet that if they had gotten to the stage of tanks 1, 3 and 4 dry, then the tank 2 normal pump would have started feeding - It happens from time to time that one of the pumps gives slightly less pressure than the others, and takes a while to start feeding (according to the fuel synoptic), even in the 4 engine case.
5t is therefore not too much of a drama (following a diversion).
However, in my book it would be quite a bold decision to continue across the very cold and remote bits of the atlantic on three.