I shoot weddings, but also a lot of architecture and studio portraits. I'm going to order a D800 for the increased pixels to push around.
But for weddings? D4 all the way, if you can afford it. Of course, as mentioned, that's a huge jump in price if weddings or sports is only a smaller part of your work.
My d700's work perfectly for weddings, but I'd take another 3 or 4 stops of ISO in a heartbeat. Plus, the framerate of the D4 can be quite necessary when capturing quick moments without having time to compose and focus normally.
Honesty, a used D3s, or even a D700, will get you everything you need for weddings. The D4 is NICE, but if I weren't going to shoot video, a D700 and the extra cash for lenses would be my choice.