On camera fill flash needs to be turned way down using -FEC. Off camera is better, but the D4 doesn't have a built in flash/commander mode (and it should still be set below ambient generally). You could get a SU800 to act as a commander, or get another SB and use the 800 as a commander (or remote triggers like PW's).
The D4 has the ability to separate EC away from the flash, but by default camera EC affects both the ambient and flash exposures. In A mode the D4 will use minimum ISO and whatever SS ignoring any minimum SS setting...this is annoying. To get around this use manual mode or S mode. To use a high SS synced with flash you have to enable "auto FP" (HSS) in the menu's and on the flash if it's remote. Also, if you are shooting backlit, then you should put the flash into BL mode...mine are always in BL mode with auto FP enabled.
If using the flash on camera I would suggest using the diffusion dome and a large bounce panel (i.e. rogue flashbender) if otherwise unable to bounce the flash.