I did it with 2 G-Blades, so also 4 devices, because one G-Blade contains two boards with 40 Chips each, but it should be no difference.
this is my startup line in the .bat file for cpuminer:
minerd --freq=800 --gc3355=\\.\COM3,\\.\COM4,\\.\COM5,\\.\COM6 --gcc3355-chips=40 --url=stratum+tcp://Poolname.com --userpass=user:password
you have to set freq to =888 instead of =800 an gcc3355-chips to =5 instead of =40.
You also have to figure out on which USB Ports youre Gridseeds are. On my PC USB0,1 and 2 are Mouse, Keyboard and Printer, so the Blades start at Port 3. But you can just try.