The White Balance Picker sample area in LR4 can now be adjusted using the 'Scale' slider, so yes it has changed from LR3:
http://feedback.photoshop.com/photoshop_family/topics/white_balance_ey edropper_tool_option
The readings you see in the LR Histogram in no way reflect the actual raw data values. The White Balance picker needs a sample area that has no raw data clipping in any channel (i.e.All four R,G, B, G2 channels 99% or less). I suspect the 99% area you are sampling has raw data clipping in one or more channels. If interested you can download RawDigger to examine the image file to confirm:
I suggest choosing a sample area that is well below 99%, such as 85% to 95%.
