According to SonyAlphaRumors, the new D4 expected in August will use a Sony sensor tweaked by Nikon engineers. They are expecting either 24 or 36mp.
They give the rumor a SR4 rating (SR5 being the highest).
This would be a major departure for Nikon, since the D3 sensor was home grown by Nikon (although the 24mp D3x sensor was based on a Sony sensor). If they had stated the D4x will be based on the alleged 36mp Sony sensor, that would seem potentially credible. However the D4 itself is more likely to be in the 18-24 mp range, and is widely expected to be a Nikon design, especially after the success they had with the D3 and D3s sensors.
Source: sonyalpharumors.com on 7/5/2011.