>...Just took delivery of a D4 to replace my D3s. Custom setting e3 Optional Flash is grayed out. I read the manual and what its supposed to do, also attached a SB900, its still grayed out. Any hints on why this is?
My reading of the D4 manual is that custom setting e3 is used to set a control mode for an optional SB-400.
My SB-800s identify themselves when attached to the D4. Perhaps your SB-900 does as well. If so, the menu item may be grayed out because you don't have the expected SB-400 flash attached to the D4.
Have you tried attaching an SB-400 to the D4 to see if the e3 menu item becomes available?