Lightroom Product Manager Tom Hogarty has posted a list of Lightroom 4 issues that have cropped up and details on how Adobe are working on them.
Included in the list are
- Point Curve Migration Issues-LR4 lose custom point curves.
- Edit In.. issues with 3rd party plugins notably NIK
- Edit In Photoshop requires 7.0-Camera Raw 6.7 RC will work.
- Temporary Reverse Geocoding issue
- Serial number issues
- Upgrade details-You can upgrade from any version of Lightroom
More detail and current progress can be had from Lightroom Journal.