I'm trying to run 4 block erupters under macminer 1.3.17 using the FPGA/ASIC miner window on 10.8.4.  Using the parameters '-S /dev/cu.SLAB_USBtoUART' I'm able to get one running but if I add the remaining 3 by adding their ID's e.g. ('-S /dev/cu.SLAB_USBtoUART, /dev/cu.SLAB_USBtoUART1') I get an error along the lines of 'unexpected commandline parameters.  I've looked at the bfgminer docs and it looks like I'm doing it correctly but I'd appreciate a fresh set of eyes and experience.  Thanks.

I forgot, I'm using them in a 4-port AC powered USB hub.  If that makes a difference.