If one is to get only one lens for Nikon wildlife, the 200-400 is the lens to get, unless one is a dedicated small bird shooter where a longer prime might be a better option.
Four important things often overlooked by people who have not used the 200-400 lens for wildlife, and who tend to see only that it's slower than the 300f2.8 or shorter than the 500f4:
1. The obvious framing convenience of a zoom
2. The not so obvious but very important advantage of the 6.4-6.5 ft min focusing distance on a 400mm lens. Max. magnification 0.27 (compare with 70-200vr 0.16, 300f2.8 0.16, 400f4 0.13).
You can almost do macro by adding a short extension tube.
3. The (limited) handholdability of the 200-400 vs. virtually no handholdability of the longer primes (even if they had VR, which they currently lack).
4. You only need to carry one lens to cover most shooting situations vs. having to carry 2 or 3 (maybe a 200, a 300 and a 500). Out in the field, this might prove much more important than any other consideration
Only reason to ever consider the 300 2.8 VR or the 400 instead of the 200-400 is if one really expects to need that 2.8 aperture A LOT, which tends not to be the case for the average wildlife shooter (as opposed to sports shooters). Things were different in the film days but in the digital era with ISO 400 quality being excellent on any camera, the importance of the 2.8 aperture has greatly diminished for wildlife shooters IMO.