A post from Dan in another forum...

If you, like me, are rushing to upgrade to LR 4, there is a bug you might want to know about. Apparently, on some but not all installations, if you import your old catalogs, LR 4 will wipe out the tone curve adjustments of your images edited under earlier versions, even if you don't update those images to the new 2012 process. One poster said that he was able to recover them by reopening the images in LR 3, but I have not seen this verified. Adobe has acknowledged the problem: http://feedback.photoshop.com/photos...ve_adjustments.

I was unable to determine how to install the fix, which presumably will come in the form of an updated version of LR, without first installing the original, if you purchased the original on a CD, as I did. My plan is to install LR 4 but not import my old catalog until the update is released. If anyone has a better idea how to handle this, it would be great if you would post it.