Do Not Upgrade to Lightroom 4, Yet
Many of you probably noticed Adobe released Lightroom 4 this week. I wanted to bring to your attention a VERY CRITICAL known bug that will impact your Lightroom 3 Catalogs. Thank you to David Swinney for bringing this to my attention.
It appears that if you update a Lightroom 3 catalog to Lightroom 4 it will overwrite the Tone Curve created in the Develop module for every image. This is regardless of the Process Version used, and it cannot be undone via History (at least reports at this point are inconsistent). Adobe is aware of this issue and working to resolve it.
My recommendation is that you make a copy of your old LR3 catalog prior to updating to LR4. For those that want to see examples of the issue you can read this blog post by Chris Marquardt. If you follow the links he provides you’ll see even more examples provided by other LR4 users.