Today, I tested the beta with some high iso NEF's from my d300.
Using the new 2012 engine, there is a very clear difference in the color noise between both raw convertors. LR4b shows almost no color noise after correcting for some WB issues and getting the exposure and brightness right, although the color noise slider is set to 0. This seems to be default.
In ACR I do the same tuning for brightness and WB, but I have to pull the slider to about 20 to get rid of the color noise.
Is it true that LR4b automatically reduces color noise?
The color NR reduction technology and implementation of ACR 6.6 and LR 4 Beta are the same. If the "Color" slider is set to 0, then color NR is off entirely.
However, in Lr 4 Beta, color NR (when enabled) is always applied at < 1:1 -- such as when viewing an image in Fit View. In ACR 6.6, this is not always the case.