It actually makes perfect sense to me that Lr 3 tone curve settings might be dropped in Lr 4: it's clear enough to me that they wouldn't give the same results anyway, so you're still going to have to rework the tone curve to accommodate the other new processes in PV 2012.
The current issue, which the engineers are working hard at, is that somehow, in the update process from LR3 to LR4, the point curve settings are misplaced. This is a current "bug" in the update process, but as I understand it, the settings have not actually been lost. I say misplaced because the engineers seem to be able to find the settings in the updated catalog, it's just that the settings are not showing up correctly in the image record.
One workaround depending on how you've handled .xmp metadata in LR3 is to take the images whose curve settings have been misplaced and read the .xmp from the file. If, like me you don't automatically write .xmp to files, you can do it manually in the LR3 catalog before updating the catalog to LR4.
As I understand it, the engineers are working on a "fix in place" for people whose point curve settings have been misplaced. Obviously, this is an unfortunate situation for people who have made extensive use of the point curve editor–note, it doesn't impact the parametric curves at all.
You might ask why the internal beta testers didn't find this issue...we are asking the same question. As you might expect, we've have many, many builds since the public beta. One of the first builds after the public one turned catalog updating back on. As far as I know, those builds of the catalog update did successfully convert all catalogs (all the way back to LR1). But somehow, it seems something in the last RC that went GM broke that tiny bit of the catalog update. We're just as shocked to find this bug on the GM as the engineers are. I just can't give any time frame for the fix.
So, if you are a heavy user of point curves, either use the workaround of writing and then re-reading the .xmp to recover the point curve settings or hold tight for a fix to be released.