I checked the Adobe site and you are correct--you can work on JPEG's abd tiff's, just as in LR. What is not clear is whether ALL the controls, including the curves, targeted adjustments, and Hue/Saturation section availble in LR Develop module are present in ACR 4.0.
I am curious why Adobe did not iclude support for it's own PSD format as well.
MDIJB