I'll look again in the manual, but for the life of me I can not find an SU-4 mode on the camera. I did notice some settings are not allowed with flash in iTTL mode.
We managed to get the protraits lastnight by using the SB-900 on the hot shoe. Some shadows could not be avoided but I shot several shots with the speedlight bounced in various directions and was able to photoshop to parts I liked into one awesome portrait.
I will get the SU-800 ordered but now I am thinking of just using the SB-900 as the commander and pick up some 600s for fill light.