>I ordered some sony cards to try out with the CF cards. I will know soon enough.
Mine came with a XQD reader & a 16 GB (I think) Sony card. I bought a pair of 32 GB XQD cards from Amazon. I use them for Large JPEG backup of the NEF images on CF.
The D4 writes VERY quickly to the XQD cards. If I wasn't already heavily invested in CF cards and readers, I would be tempted to go that way.
I wish Nikon had picked ONE standard, and doubled down on that slot format. I'm betting the D5 (whenever it is released, 2016 or whenever) will have dual XQD slots.
The shooting buffer (whatever it's called) is ginormous, too. Just for fun, I set the D4 on continuous high, AF-C, NEF to XQD and JPEG-L to CF, and blazed away at gulls on the wing. About 40-some images later (all in sharp focus, of course), the D4 BEGAN to slow. I think it pooped out at 60-some exposures before it had to pause to write. (That light went out pretty quickly, too.)
The D4's no slouch at writing to CD cards, either.
And it's fast ENOUGH for my purposes. (I grew up in that old school where I was trained to wait for The Decicive Moment instead of blasting away at 10 fps while hoping that the camera MIGHT have tripped the shutter at the right moment.)
I hope you love your D4 as much as I've grown to love mine.