So Alex; which one will you buy, or will you buy both?
Sorry for the slow reply. Was finishing the huge Wetpixel review. The conclusion should be online soon.
After trying the D800 in January, I bought the D4. The D800 is amazing, but the D4 is a better fit for the projects I am working on and the amount I shoot. I got it in March and like all my cameras (and lenses) the first thing I did was dot the i in red ink so I can easily spot my gear on busy liveaboards etc.

But for the majority of underwater photographers I think that the D800 makes more sense, not for the 36 million reasons, but the 3000 reasons. I have tried to write the reviews (both the short UWP one and magnus opus Wetpixel one) from an objective position, rather than from my personal perspective. So both reviews comes down in favour of the D800, while stating the reasons that some pro UW shooters will clearly prefer the D4 for the type of photography they do. I am in that camp.
That said, I will be shooting the D800 this year too. In many ways it is the more interesting camera to review because there are more unknowns. The D4 is an improved D3 with video. The D3 worked very well underwater. The D4 is better. Case closed. The D800 is more of an unknown. Not least how much resolution will we be able to achieve with it through water and behind a dome port. I honestly don't know the answer. Also is the DX mode useable underwater, or if the viewfinder reduction too much of a limitation. Is it better to always shoot FX and then crop in post? Unless you are doing a contest.
I also don't know whether the D800 or D800e will be better underwater. Will shooting through seawater act as a low pass filter? Or will patterns like fish scales be a moire mess?
It will be fascinating to find out.
I felt there were too many unknowns to buy a D800 or D800e yet.
And then we have the rumoured third FX Nikon this year (D500/D600?). Cheaper than the D800 and probably midway between the D4 and D800 on resolution and pixel pitch. One rumoured mock up suggests it could be small enough to fit in D7000 housings! But then again, it may never exist at all. If I was Nikon I would concentrate on building D800s (and to a lesser extent D4s) to meet the demand - there are still long waiting lists.
Also I should delete Nikon Rumours from my bookmarks on Safari! To many ifs. Although what worries me most about it is that it may also spell the end for pro-specced DX Nikons. Maybe from now on the D7000 tier will be the upper limit for DX, which would be a great pity for underwater photographers.
Alex