Just heard this morning of a problem some people are experiencing with Lightroom 4. If you open an image in LR4 to which you previously applied a custom point tone curve edit in LR3, you may find that LR4 dumps all your Tone Curve edits, and simply applies the default Tone Curve for the camera. The Parametric Curve seems to be unaffected: it’s only the Point Curve which seems to cause a problem. Adobe are aware of the bug, and are promising a fix soon.
Fortunately, if you re-open the image in LR3, the custom point curve seems still to be there, so the edits you applied in LR3 are not lost.
The issue only affects images previously edited in Lightroom 3: if you have never used Lightroom before version 4, there should be no problem. However, it might be worth sticking with Lightroom 3 for the time being until Adobe have ironed out this bug.