Last time I looked the transfer starts when the centre wing tank has reduced to 36,470kgs.
If the transfer fails to take place then if your gross weight is less than 378,000kgs you have, as usable fuel remaining, wing tanks 1 and 4 and main tanks 2 and 3 down to 7,700kgs in each, by which time you must have landed to maintain c of g limits.
If your gross weight is more than 378,000kgs at the time the transfer fails then you have, as usable fuel remaining, everything in wing tanks 1,2,3 & 4. Land before main tank 2 & 3 are empty to maintain c of g within limits.