The SU-4 mode turns the SB700, SB800, SB900, and SB910 speedlights into simple optical slaves. They will fire any time they see the pulse from another speedlight, or bright flashing light. I use the SU-4 mode a lot with my herd of twelve SB800s, and have had all of them start firing wildly when a police officer drives into one of our sets with all of his strobes on the light bar firing.
The SU-4 mode will not work with other speedlights that are in the CLS mode, as it will trigger on the preflash pulse that occurs prior to shutter opening. Then, when the image capture pulse occurs while the shutter is open, they will not have had time to recycle and fire again.
Hope this helps a bit.
HBB in Phoenix, Arizona Nikonian Team Member
Photography is a journey with no conceivable destination.