While I read all the manuals, it takes some practice to refine your results when shooting with any camera gear.
My first recommendation is to choose manual sensitivity on your microphone. When the microphone is set to Auto, and your subject stops talking, the Auto setting will crank up the gain [volume], introducing noise or hum in the background.
After much trial and error, I have noticed that if you move above seven on the microphone setting, you will introduce noise.
My recommendation is to buy an external microphone. I have two. I usually record all my interviews with two cameras. One camera that is pointed straight onto the subject has the shotgun Røde Video Pro microphone on that camera. The second camera, left or right of the primary camera, has a wireless lavalier Shure FP1 microphone. WL183 (Omnidirectional): Recommended for general purpose sound reinforcement, recording, or remote monitoring applications. Low handling noise. Pickup angle: 360 degrees. Clip on the subject and keep it close to their mouth.
Both of these microphones work with DSLR video cameras. First, they have a stereo 1/8″ plug to put the mono sound on both channels. Next, both of these microphones let you boost the gain so that the sound coming out of the microphones, so you do not have to turn your gain up too high on the camera.
The Røde microphone has a +20 dB gain. I recommend using this setting and then listening through your headphones to adjust the input gain level. By doing this, you will avoid most of the time going above the seven gain level on the Nikon D4.
On the Shure FP1 lavalier, there are transmitter and receiver gain controls. My mistake early on was I only adjusted the receiver. If you turn both of these up, just like with the Røde microphone, you can avoid going above seven on the gain setting with the Nikon D4.
If you continue to find that you need more than seven regularly, you need to get an amplifier on the line to increase the volume so you can keep that number lower than seven on the Nikon D4.
You may want something like the Beachtek DXA-SLR PRO HDSLR Audio Adapter, which will let you increase the volume so you can keep the gain level lower on the camera.