One of the cool new features in Lightroom 4 (LR4) is the added control to the Adjustment Brush and Graduated Filter. The more features and controls Adobe adds to this tool the better it gets. Now in LR4 Adobe has added a local White Balance control which is a huge benefit for photographs shooting in mixed light sources. A new HIghlights and Shadows control is also added. The third set of new controls added are Noise and Moiré sliders which are a welcome addition for those who want to remove noise in selected areas of your photo when its not needed in the entire image.
Truly the biggest addition here in my opinion is the added White Balance control. The Noise and Moiré controls are great too but I've had too many photos that were taken indoors and contaminated by the outside light from a window. Previously in LR3 you could only balance for one light and that presents a problem. Also, if you're photographing a wedding there is no control over light and you're at the mercy of what is falling on your subjects. This is not going to be a local adjustment you apply to a hundred photos but for the real important ones, you can make the fix when needed.