I too have both cameras. I don't need both but will probably keep both.
I haven't noticed any difference in the images with regards to vibrancy. I suppose there could be a difference but it must not be by much as it hasn't caught my eye yet.
The D800 has better dynamic range at low ISO settings but anything above ISO 400 the D4 begins to surpass the D800.
If you can shoot at ISO 100 then you can pull an amazing amount of details out of the shadows with the D800. From my experience, this is one of the most dramatic improvements over my old D3s.
However, I find that I am adjusting the ISO setting on the D800 more than ever before. I'm trying to keep it low for better images but it is sensitive to any movements in the camera if the shutter speed starts to drop at all.