I doubt SoftwareSerial is going to do the job you need. In fact, given the strict conditions that would have to be met for there to be even a hope that SoftwareSerial would work for more than 1 stream, I would say with confidence that it won't work. Now the UART Crossroads suggested, that's is worth your time. Two of those added to a mega would be 8 UARTs. Four of those coupled with some RS-422 driver chips and you are in really, really good shape. Then you can use any Arduino board you like.