Help to get Arduino UNO with 4 SoftwareSerial transmit only ports
As Chris Stratton suggests, multiple instances of SoftwareSerial may cause issues, particularly with pin-change interrupts. 

I adapted SoftwareSerial a while back to have a "send only" version. This can be downloaded from here - 6 KB.

One normal SoftwareSerial and 3 instances of the "send only" may possibly help. I haven't tested it.



Chris's suggestion to modify SoftwareSerial to allow pin number changes on-the-fly may be even better. SoftwareSerial can only write to one pin at a time as it blocks, and thus having 4 instances of it doesn't really achieve anything.



Another possible approach would be to use a multiplexer chip. I have a page about that. You could have a single SoftwareSerial instance, and then use the multiplexer to forward its data to one of (up to 8) pins.



You would need pull-up resistors to keep the other pins high (idle) when not in use at a particular moment.