John, Mongo had a D800. Then had opportunity through a friend to update to D800E for a couple of hundred dollars - so did so. Still own the D800E. In the interim, about a year and a half ago, there was a sale on new D4s models. So, Mongo bought a new D4s thinking it would be the thing to have that would please him. Well, it did and it didn't.
As a result, Mongo has kept both cameras because they do their best at different things.
the D4s is brilliant to use, great quality etc but in Mongo's mind is largely a studio camera OR one for full size macros or full size landscape etc. In short, it is no use and not a patch on the D800E when you need to seriously crop an image. Thus, for long wildlife shots, Mongo will always use the D800E in preference to the D4s unless he is right on top of the subject and little to no cropping is required.
The noise performance is not that great to write home about with either camera. Certainly not that different to each other in virtually all conditions Mongo has used them. So, that should not be a deciding factor.
If Mongo were you, he would seriously think about the predominant use you will have for the camera before making any decisions. Certainly, hire one for a weekend if possible and run it through the type of photography and conditions you would ordinarily encounter.