While I read all the manuals it takes some practice to really 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], which will introduce noise or a hum in the background.
After a lot of trial and error I have noticed that if you move above the setting of 7 on microphone setting you will introduce a noise.
My recommendation is to buy an external microphone. I have two. I normally 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. On the second camera that is to one side or another has a wireless lavalier Shure FP1 microphone on it. 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 close to their mouth.
Both of these microphones are designed to work with the DSLR video cameras. They have a stereo 1/8″ plug to put the mono sound on both of the channels. Next both of these microphones let you boost the gain so that the sound coming out of the microphones is boosted so you do not have to turn your gain up too high on the camera.
The Røde microphone has a +20 dB gain. My recommendation is to use this setting and then listening through your headphones adjust the input gain level. By doing this you will avoid most of the time going above the 7 gain level on the Nikon D4.
On the Shure FP1 lavalier there is on the transmitter and receiver gain controls. My mistake early on was I only adjusted the receiver. If you turn both of these all the way up, just like with the Røde microphone you can avoid going above 7 on the gain setting with the Nikon D4.
If you continue to find that you need higher than 7 on a regular basis then you need to get an amplifier on the line to increase the volume so you can keep that number lower than 7 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.