What is a neutral density filter? Here is the Wikipedia entry for an ND filter - "In photography and optics, a neutral-density filter, or ND filter, is a filter that reduces or modifies the intensity of all wavelengths, or colors, of light equally, giving no changes in hue of color rendition. It can be a colorless (clear) or grey filter. The purpose of a standard photographic neutral-density filter is to reduce the amount of light entering the lens."
In other words if you are flying your Phantom 4 for photography or videography purposes; or, any drone for the matter, outside on a sunny day then you should be using a ND filter to get the best results possible. The ND filter will help with rolling shutter; as well as, help create a more natural cinematic motion blur for your videos.
A good rule of thumb to follow when using a ND filter is your shutter speed should double your frame rate. As an example, if you are shooting in 1080P @ 120 FPS then your shutter speed should be set to at least 240. Of course it is all preference but that is the norm that most videographers will tell you to follow to achieve the best results.
I am currently testing ND filters from Polar Pro with my Phantom 4; but, due to crummy New England weather currently the sun isn't shinning bright enough for me to test all my filters to see the differences with and without the ND filter on my Phantom 4. I have used the Polarizer filter which is great for flying around water or snow; but, it is better suited for bright days as well. The ND4 filter is good for gloomy days when the sun isn't shining as bright, but only being able to test 1 of the 3 filters is sort of meh.
For more information on the Polar Pro ND filters, checkout the Polar Pro website.