So the question is (to me), can a Nikon D750 replace a Nikon D4? Yes!
It is safe to say that the D750 is up to the task of wedding photography. The lighter body is welcomed and the deeper grip is one of the best I have experienced. But, what really floored me was the autofocus speed. Oh my! You really have to experience it. It focuses down to -3 EV. So, it basically sees in the dark. It is really remarkable. It focuses at the limits of what humans can see when the lights go out. It would grab focus in the middle of a dark dance floor and find contrast in the absolute worse lighting conditions. I don’t think I missed one shot all night.
The High ISO is crazy great. The AF speed is insane. The tilty LCD screen comes in handy when you want to grab a high shot looking down on the dance floor. It is a great all-around tool that will (and has) replace many flagship bodies in the bags of a lot of wedding photographers. I could be very happy with 2 D750 bodies.
I think Nikon really hit a homer with this camera. It is a great camera for both a fast-paced wedding as well as your child’s softball game. It balances fine with a variety of lenses and it is comfy and light. The video is fantastic as well.