It is a pity that the D4s, a high level camera from Nikon doesn't offer an AF assist red beam from the on-shoe flash.
When used with a red beam flash, such as a 910, there is no way to have the AF assist red beam lighting up, which makes photography in extreme low-light conditions very difficult. While cheaper cameras such as D800 has this ability.
I know that the D4s doesn't have a built-in flash but Nikon should have foreseen the function to send the command for the AF assist red beam to on-shoe flashes.
I wonder if the D5 has this function embedded.