A few more observations on the D750. Color richness is not quite as robust as the D4, to my eye. Noise at high ISO preferable in D750 than D4, particularly in JPEG. In RAW D750 can show a bit more apparent noise, but it's more grain-like and less clumpy than the D4. And the entire image is sharper. Talking high here- testing 10,000-12,800.
One option for dealing with high ISO noise with RAW files is to try out Nikon's NX-D software. Not too elegant but it works, and NR is seriously good.
In terms of IQ, at least until 6400 I still trust the D4 more, it's looking like the files have a bit more juice to them. Though operationally love the D750 shooting hand-held and with primes- so comfortable due to small size, light weight, and really good grip.
I was considering selling the D4 for a 2nd D750, but at this point my own results are telling me to hold onto the D4- each body has the advantage in different situations.