... plus, as EFLI (the person!) said, the 'fixed'part of the fault current path ('loop') all the way back to the transformer at the local sub-station.And the reason the flex is unlikely to effect it is because the short length of the flex (1-2m) and the difference in diameter (2.5mm² vs 4mm²) is likely to make little difference to the EFLI compared to the 'fixed' part of the installation at 6mm²/4mm² which runs the distance from the CU to the cooker outlet?
So, the new little bit of cable will make very little difference to the EFLI (the impedance!) or the potential 'fault current' in the case of a dead short. What one has to do is ascertain (by measurement) what that fault current would be (as I said, virtually the same with or without the new little bit of cable) and then, by calculation, determine whether the new bit of cable could 'tolerate' that current for the very brief period before an MCB of fuse disconnected the supply (without melting or otherwise being damaged) - which, as has been said, it very probably would be able to do.
Kind Regards, John
Edit: too slow typing, yet again