Thanks, bigduke6.
I believe there is also logic to give you the tank-to-engine message on the ground prior to engine start with tanks 2 or 3 containing up to 500 kg more than tanks 1 or 2.
Interesting. I can't find any reference to it in the wiring schematics.
The logic in the manuals does mention that the inboards have to be full to start the logic process. i.e. if you manually loaded the tanks M1=10, M2=11, M3=11 and M4=10 and configured the aircraft to burn the fuel from the inboards, in theory, you would not get the message when the tanks were equal.