Canon hasn't answered the D800/E yet.
At the risk of piling on here, I will pile on.
If there is any question, it ought to be why hasn't Nikon answered the 5DIII yet.
Been there, explained this before, but here goes:
The 5DIII was targeted to a real audience: wedding and event photographers. They wanted better autofocus and clean high ISO performance. Go back and read some of the threads from when the 5DIII was introduced and you'll see glowing comment after glowing comment from these photographers about how much they liked the 5DIII. The camera is a "must have" for many wedding photographers because it gives them a competitive edge – they can get shots that other photographers can't easily get.
This is a real market and in fact, it is one of the few large professional markets that exists today.
It so happened that in meeting that market, Canon produced a camera that many of us not in that business still find to be a highly desirable product. Take a look at the Amazon sales rankings. It's incredible that the 5DIII consistently ranks in the top 10 against cameras that sell for one-fifth its price. Having met the needs of a specific audience, Canon was able to expand the base to enthusiasts like myself (who are still a little shocked that we bought a camera this expensive).
Nikon introduced a camera designed to meet the pent-up demand of its buyers, who had suffered for years with cameras that had fallen behind in resolution. I think they resolved to fix that. Unfortunately, the D800 doesn't seem to have found a broader market. Once the pent-up demand of Nikon users was met, where could Nikon go for additional sales? Landscape and other photographers who want high-megapixels and slightly increased dynamic range simply do not constitute a large professional base. (How many professional full time landscape photographers live in your town? I live in the Midwest and I'm not aware of any in my city. Compare that to wedding photographers, which seem to be coming out of the walls around here.)
So, please, somebody give me a good business reason why Canon "needs" to answer the D800, because frankly I can't find one.
By the way...I still can't quite figure out why we are talking about the D800 when this thread is about the D4s – Nikon's low megapixel flagship. If the D800 sensor is so awesome, why is Nikon so uninterested in using it in its flagship?