Designed for the Phantom 4 Camera, it replaces the UV filter that is pre-installed on the camera. Helps reduce the amount of light entering the lens to reduce glare and lower the Exposure Value.
Especially useful when shooting video to reduce shutter speed to double the frame-rate to introduce cinematic motion blur.
Put more simply, the ND4 filter helps avoid overexposed photos.
The ND4 filter allows 1/4 of incoming light through, equal to a 2-stop filter.