If you really think about it, Nikon will need to score the same shock it did when it announced the D3X with its 24mp sensor, and the D800 with a 36mp sensor. Simply incorporating the 36mp D800 sensor into a D4x body isn't going to do it, even if Nikon can add some processing speed so the drive can travel faster than the slow 4fps of the D800. To justify the huge price difference between a D800 and D4X model something bigger is going to have to be offered.
I'm speculating it will be in the 54-56mp range. My reasoning is that, first of all, this will be a shocker size, something that Nikon will need to accomplish for its PR value. After all, there is already a 41mp smart phone camera out there making a 36mp full frame camera yesterday's news.
Second, 54-56mp isn't so large that the camera will be bogged down transferring information from the sensor to the card. A larger memory buffer, similar to what Nikon added to the later D3s and current D4 will be necessary to improve the processing speed and consequently the fps of the camera, and ,although expensive, will be justified in a high end, high priced camera. Even so, I'd look for 4-6fps as a maximum drive speed.
My third reason for choosing the 54-56mp size is that Nikon is not ready to make its own D4 obsolete. A smaller 36mp sensor combined with beefed up processing speed is going diminish the differences between a D4 and D4X. There was never a question of D3 or D3X because the gap between them clearly defined their particular uses. For a similar gap between a D4 and D4X the megapixels will need to jump to where the speed of use is different enough not to compete, thereby turning an "either-or" decision between the two bodies into a "both-and". That feeds better into the strategic nature of marketing.