WIth just one controller, they'll all have to operate at once, a train approaching from the B side will trigger A and B, and keep A and B going until the train clears the A side. Likewise a train from A will trigger A and B, and kee both going until the train clears B.
Anythign else requires two controllers, one for A and one for B, with the sensors interleaved, such that the sensors that start B would have to be on the A side, but before the sensors that stop A. SO a train from the A side would tigger A, pass through the crossing, trigger B, and then as the last par passed some point between A and B, A would stop but B would keep going until the last car passed B.
The Logic Rail controller uses 2 sets of sensors on each side of the crossing, that way it can tell which way the train is going. The Circuitron controller is a much simpler device, with just 2 sets of sensors, and you have to somehow tell it which way the train is going, so the sensors on the approach side become the activate sensors and the one on the opposite side are the deactivate ones. The Azatrax one looks like it can be used either way, one sensor on each side, or two sensors on each side.
It seems liek what you need is somethign with multple sensors on one side (where there are more tracks) with just one sensor on the other side (where all the tracks have converged to just one). You really need to call one of the controller vendors to see what they suggest. They may or may not have the logic in them to do this. This would be a perfect DIY thing, because you can so that if th sensor on track 1, 2, or 3 is tripped, start the flasher until the sensor on track 4 on the far side is tripped, and if the sensor on track 4 is tripped first, activate the flashers until any one of the other 3 sensors is tripped, so it won't matter which route the train takes.
--Randy