I have big collection of pictures and catalog created in LR2.7. I try to open this catalog into LR4 and have message : An unexpecting error . Try another catalog"
Lightroom should be able to do this. When selecting to open a catalog from an older version of Lightroom, LR4 will bring up the "LIghtroom Catalog Upgrade" dialog and inform you that it is about to perform the version conversion task.
As you are getting an error, this tells us there is some problem with the catalog and LR4 is having a hard time reading it. Open the catalog in LR2 and go to Catalog Settings. Under the General tab, click the "Relaunch and Optimize" button. Close LR2 and try LR4 again.