Thanks to someone over in Japan, we can now see the first hands-on photographs of the soon-to-be-available Sigma DP2 Quattro. And I just found another one here. Thanks to Google Translate, we can get an idea of how the camera performs.
The gist of it is this: battery life has improved considerably over the DP2 Merrill, auto-focus has improved a little, write speeds have improved, too, the JPEGs are much better, high-ISO performance has improved perhaps as much as two-stops, the rear screen is better, and overall resolution is slightly higher (although not significantly higher).
It would seem that at ISO 100, there really isn't any difference in image quality between the DP2 Merrill and the DP2 Quattro. The Quattro seems to give a better overall user experience while also improving high-ISO capabilities.
Right now the DP2 Merrill can be had (if you shop around) for almost half of what the DP2 Quattro is listed for. I'm not sure that the improvements are worth the extra money. But if you do think so, the Quattro should be available in the U.S. sometime next month.