skyjumper:
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.
I found in old posts that you worked with that chip. Did you success on your project ?
Can you give us some schematics, or elements that will help me to realize it ?
Best Regards
I also found on the net : http://hackstrich.sarahemm.net/MultiSerial_Shield
May be it can be useful. Just have to remove Max232 chip and use max488. But I've to design it...